OK - after spending all day on this, I came up with a solution that works for my situation. I'll describe it here for others, but by no means do I expect this will work in all scenarios. However, if you do find this useful *and* you find a way to improve upon it, please share.
Basically, I found that I had to setup two different email fields (one hidden) on two different form pages. Then I had to use hidden radio buttons to say whether we're dealing with a new user or existing user. Then I had to modify my Registration Add-On settings. Finally, add some hooks and javascript to tie it all together.
1. On a multiple page form, prior to including any other Email field, include a single line text field. Let's say page #2, and call the field "Enter Email" and make it required. Let's assume that field ends up with a form input name of "input_10". This is where the user will enter their email address. I'm pretty sure you can put some formatting validation on that field through GV, but I haven't done it yet. If not, there's always some javascript you could use.
2. Save your form.
3. Just below "Enter Email", make a radio button field. Let's assume that field ends up with a form input name of "input_11". Let's give that field a label of "is_registered". For the radio buttons, give one a value of "Yes" and one a value of "No". Make the "Yes" the default. Make the button required. On the Advanced tab, change Visibility to Admin Only and check Allow field to be populated dynamically (not really sure if that's necessary or not). Leave the parameter blank.
4. Save your form.
5. At this point, you should *not* have included the standard Email form field anywhere before step #1. But now it's time to add it. However, you need to add it on the next form page. So go ahead and add a page break, if you don't already have one.
6. Save your form
7. On the new page, add the standard Email field and label it "Your Email". Let's assume that this field has a form input name of "input_15". Make it required. On the Advanced tab, you have to leave this field visible to everyone, cuz this process doesn't work with it hidden. However, we don't want people to see this field because they already entered their email address in the step #1 field. So we have to throw some CSS on it to hide it. In the CSS Class Name field, enter "iamhidden". We'll create the CSS in a bit.
8. I left the "Allow field to be populated dynamically" field unchecked because I couldn't figure out a good way to use it, but I tried. This is an area that someone might really be able to improve upon. Because instead, we have to use some javascript to populate this "Your Email" field - we'll to that in a bit.
9. I was finding that I needed another form field on this page. That may or may not be true. Either way, you don't want users looking at a blank form, so you'll have to add something. Your form scenario probably lends itself to that anyway.
10. Save your form
11. Jump over into your User Registration Settings/Feed (I'm assuming you already have this setup to some degree). Set the username field to the "Your Email" that you created; set the Email Address field to the "Your Email" that you created; set password to "Auto Generate Password"; set all other fields as appropriate.
12. Check the box for the Registration Condition. Set the drop-downs to "is_registered" "is" "No".
13. If you're automatically creating posts, you'll probably want to enable "Set As Post Author". But keep in mind that the post author will only be set to the user when it's a newly registered user (due to the fact that we just placed the conditional in step #12). So if you check the box, you'll need to add another hook later on to set existing users as post authors. The drawback to that is you'll be setting post author without necessarily authenticating the user, which could have some negative consequences. I'll show you the code to set the author, but it's kinda a separate topic.
14. Save the settings
15. Open your functions.php file and add the following code. This code will get executed when the user navigates from form-page-to-form-page. This code will get the email address from the field we created in step #1 (Enter Email) and look at Wordpress to see if there is an existing user with that email address. If the user doesn't exist, the radio buttons (from step #3) are set to No. This in turn tells the Registration Add-On to create a new user. If the user does exist, the radio button are set to Yes and the registration process won't get in the way of adding a new entry because of the conditional we set in step #12:
http://pastie.org/4596054
16. As described in step #13, you may or may not want to add the following code to your functions.php file. This is the code that will update a post's author when your user isn't logged in. Again, there are some real security downsides to this (like other people possibly being able to see someone else's post). So if you're gonna do this, you really should know what you're doing. Otherwise, don't.
http://pastie.org/4596080
17. Save functions.php
18. Open the file you're using to style your forms. As mentioned in step #7, we need to hide the "Your Email" field. Add the following to you stylesheet:
http://pastie.org/4596116
19. Finally, we have to add the code that will copy the email address from "Enter Email" to "Your Email". We do this via javascript. So open main.js or whatever file you're keeping your scripts and add the following. Be sure to replace the numbers with the appropriate numbers of your actual form and fields:
http://pastie.org/4596135
20. Make sure everything is saved and then test.
Hopefully I haven't forgotten anything. I really don't have time to support issues on all of this, so it's best to understand what you're doing here.
And maybe soon, the fine people at rocketgenius will just implement the (Register if <email_field> is "unknown in DB") condition in the Add-on.
:)