It seems that the Note Off command does two things to OPL samples:
1. It releases the instrument envelope loop point, which is fine
2. It causes the "decay" part of the OPL sample's envelope to start
I'd like the ability to use Note Off without #2 happening-- I'd like the instrument envelope to be affected as usual (as they are for regular samples) but not cause the somewhat useless OPL envelope to begin decaying. I tried making the decay rate really slow but it still decays nonetheless.
Is this possible? It seems weird for Note Off to be overloaded in this way.
No, it's not possible. And it's not weird but consistent - Note Off always affects both the instrument itself and its assigned sample slots. With normal samples, it releases the sustain loop of that sample, and with OPL instruments it releases the sustain point. There is no "release just the instrument sustain but not the sample sustain" option.
Anyway, to achieve what you want, you'd have to set the release stage of the envelope to 15, not the decay stage. Or, if you usually hit Note Off before the sustain point is typically reached, set it to be the same values as the decay stage so that the instrument keeps decaying at the same speed.