@DeeringAmps: AmpliTube 3 is not registered as VST3, but as VST (implies 2?). Purchasing AmpliTube 4 is no option...
@JonD: the interrupt idea sounds reasonable, but the frequency is strongly influenced by the selected buffer size (about 1 Hz at maximum / 10 Hz at minimum buffer size). Nevertheless, I checked latencymon, but again with negative result:
Generally, I decided not to pursue this issue any more. The strategic decision is to switch from AmpliTube to TH3 Cakewalk Edition, and my efforts just aimed at a smooth transition where I could optimize the new TH3 sounds bit by bit whenever I like and play with the old sounds in the meantime. Now, the optimization of the new sounds was a bit more urgent and is now 70% done, and I will finish it next weekend the latest.