Just tried this via DOSBox ECE SVN r4007, and it doesn't fare any better at Wizardry 7's perverted use of the OPL2 as a DAC. Is this a DOSBox mixer limitation or an OPL emulation limitation?
It's been a while since I last checked, but most likely cause is that the OPL chip emulation code is told to render several samples per call, so it's not the fault of the chip emulation itself.
Emulating PCM output would require to have the register writes to affect each output sample immediately.
Related to this, I've heard some weird sound issues on at least one Sierra game, like a sound playing for a brief moment at incorrect volume/octave/pitch or something, which only means a block of emulated sound is generated while game is in the middle of updating the chip registers. Fortunately, this is not an issue on most of the OPL stuff.