Yes, OPL2 or OPL3 could be useful in this case, too. I also saw some 8-bit computers emulators with this type of sound and although it wasn't perfect, it was useful and still much better than PC speaker.
But I think that it would be much easier to modify the TEMU output from the Covox DAC to the Sound Blaster DAC than to write a completely new sound emulator. I think it's just necessary to initialize Sound Blaster and send the TEMU sound data to some other addresses than Covox, but I can't figure out how exactly.
I also found an assembler source code of a small ZX Spectrum emulator which uses the Sound Blaster DAC to emulate a similar 3-voice square wave sound chip, but unfortunately it didn't help me. Probably it's a work for someone who is better in assembler programming or someone who is familiar with sound emulation.