It can be tricky since OPL3 is not a MIDI compatible device. When you just 'set it as the main playback synth within Windows', many processes hidden from your sight occur. Actually, Windows OPL3 driver uses some kind of an OPL timbre bank to translate MIDI into OPL compatible data.
I am not sure Creative's play.exe has this kind of functionality. If you point at where to get it (the PLAY.EXE), I will look into it further.
Meanwhile, why wouldn't you just use my PX player for your purposes? If you select SBAWE32.ADV driver, it will play through EMU8000; if you select SBP2FM.ADV or WSS.ADV, it will play through AWE32's OPL3 chip. Although, PX player might be incompatible with some (seemingly rare) MIDI files.
Here you can get fantastic wallpapers created by a friend of mine: patreon.com/Unpocodrillo