I settled with VOB InstantWrite for now (from their InstantCD+DVD package). I also tried various versions of DirectCD, InCD and some standalone programs. While directCD is not that bad IMO, it tends to have a lot of compatibility problems with other applications/drivers. And InCD is just not on par yet and also have issues -- it's even unable to UDF format a plain CDR and doesn't support UDF incremental and UDF/ISO conversion. As for standalone UDF programs, they aren't as convenient and flexible for this task.
Moreover, InstantWrite doesn't conflict with any ASPI layer, just because it doesn't use one (only main application is able to switch to system ASPI). Instead it uses its own low-level ASAPI library (compatible with EAC, btw). Even though I don't use UDF that often myself, and there might be issues I haven't experienced yet, I'm quite happy with the current choice so far. So, I suggest you try it out!
BH
PS. Runs fine here on my win2K with Adaptec ASPI 4.60 , VOB ASAPI, VSO Patin-Couffin engine, and Nero ASPI and Jukka Poikolainen ASPI layers installed at the same time!

B)