[whpalmer4]

Not really, no. The sync features are designed to share the entire document.

I presume you have two licenses for OmniFocus, so you could run entirely separate databases for you and your assistant, and just export the project in question from your database and import it into hers. Or do you need to both be tinkering with that shared project frequently?

Another option, if you both need to have access to common data, but you want to keep your own stuff separate: set up an alternate user on your Mac. Run OmniFocus there, set up to sync with your assistant via MobileMe. Only put the shared stuff in that database. Everything else, do in your own database, in your primary user, with no syncing set up. Shifting things between the two databases will not be terribly convenient, but neither will it be impossible.