I do similar things with just stuffing data in a hidden field for long stuff like that. I don't see any reason to bother trying to encrypt it; if the user wants to put crap data in, they could already have done that in the first place. Just have to validate it on submission.

Or, if you really do want to encrypt it, you don't need any special CGI-ish thing for that. Just AES/Serpent/etc the serialized string (whatever format it may be in) and dump that in the form field. Gen up a new key for each session, and just use that for all their forms, should be plenty secure enough.

An additional option would be to instead of storing in the DB just for the session, create a new little opaque key for each run through a form (a sha256(time()) type construction would be fine; just bitstirred pseudorandomness) and use that and the session for storing it. That way you could have multiple stored saved bits per session, and you just need to stick that key in a hidden field in the form.