I've had to deal with a similar situation like this before. In the end we just designated one developer (me) as the version control point of contact for the 3rd party.

The 3rd party would email me a zip file of their project files every day and I would do the checkin for them. I setup a separate project workspace and svn account for them and would unzip the files into that workspace overwriting what was there and then do the checkin under that account.

It's wasn't the most fun to have to do every day but sometimes it's more important to just get the work done.

One plus was that it did help me review their work to make sure they were not checking in bad code and data that would break the build.