ABC is a fairly-widely supported format, there certainly exist ABC to MIDI and ABC to Lilypond. If there doesn't already exist a web gateway where you can paste in a chunk of text and have a Midi file spat out at you I'll try and set one up tonight.

That still needs me to have a MIDI player. Do those come as standard on (say) Windows these days?

Yes. Windows Media Player handles Midi, as do QuickTime, RealPlayer, and just about all the others. On my desktop at home I have to use timidity, but that's just because my sound card drivers are iffy.