You probably don't know this, but Sylverant was originally based on Tethealla (hence the naming connection between the two, if you noticed it). However, before I ever released anything on the Internet, everything was completely rewritten (hence the lack of PSOBB support), mainly targeted at PSOv1 and PSOv2.
I believe that the reason that challenge mode isn't supported on Tethealla has more to do with the fact that the code doesn't handle any of the challenge-related packets, not so much the missing quest files. I'm pretty sure that somewhere I have all of the PSOBB quests (and I'm sure Sodaboy did too), but they wouldn't be of any help without the support written in the code for handling the various challenge-related packets.
Having said all of that, I'm not sure that anything in the Tethealla code would be of much use with regard to Sylverant at this point. I spent a good amount of time studying/documenting how things worked when Sylverant was still just a fork of Tethealla, so I'm pretty sure that given enough time I'd be able to implement full PSOBB support into Sylverant. The problem is, of course, finding the time to do it. At this point, I wouldn't expect me to have the time to work on it for a while, unfortunately.
There's at least one group of people that were looking at rewriting/expanding/working with Tethealla still. I don't know what kind of progress they have made or even if they're still working at it at this point...