I fixed some bugs and I finally figured out how sessions and tracks really work! Now there should be support for both single-session and multisession discs.
In the ISOBurn utility, I added a property page that allows you to modify the write parameters. However, note that not many of them should be changed (the Multisession field is a notable exception); if they are, the disc may not burn correctly. (For example, changing the mode from Mode 1 to Mode 2 would produce garbage data on your disc.) However, if you feel the need to experiment, go right ahead!
Note: DVD+R/+RW are very different from their "minus" counterparts (and from CD's). There is no "erase" function for them and they seem to only have one track and one session. So writing to a DVD+RW them will overwrite any previous data. (Non-erasable DVD+R discs probably won't work yet unless they're empty.)