After boot (which includes any System/INIT loaded drivers...) I have the following drivers installed:

.Sony, .Sound, .Sony, .AIn, .AOut, .BIn, .BOut, .MPP, .ATP, .ASYC00, .XPP, .netBOOT, .XPP, .EDisk, .AppleSoundInput, .SLM, .DSP, .ipp, .CTBIn, .CTBOut

.ASYC00, .SLM, .DSP and the .CTB* drivers are possibilities. Presumably .CTBIn/Out is communications toolbox...

The loaded driver's device control entry should indicate whether a given driver exists in ROM or RAM, but my driver inspection tool isn't sophisticated enough to decode that yet.