No, I don't think this is possible. We used to use Gravity Forms with our Wordpress site, and I learned that Marketo only could prefill data if we used its forms on its landing pages. Hope that helps! I know this was asked a long time ago, but maybe someone else has the same question.
Allison, this question is actually different. The OP is using Marketo LPs and Forms. But they are sending from Constant Contact, which means the built-in lead-associated links you get with a Marketo-generated email won't be possible. However, you can use Marketo pURLs in Constant Contact emails, which will prefill on Marketo LPs (from info in the Mkto db, of course). You're basically using CC as you would use a postcard mailing. pURLs have their own hurdles but those can be worked around.