Key quote from the thread:

I have contemplated at least adding an optional linear mode in MPTM, but the problem is that the sample volume range is identical to the OPL volume range (well, almost - OPL only goes from 0 to 63 rather than 64), meaning that there would be some inevitable loss of precision.

The key limitation is on OPL's side. If you were sliding MPTM volume from, say, 64 to 60, all of these would still translate to the same OPL volume, so you would be wasting a lot of possible volume values and couldn't use them for the detail that is required in the lower volumes. Without extending the total volume range in MPTM first (e.g. 0...255 or so), this is simply not useful.