I wanna speedrun desert bus, but Sega CDs are over $200 to get anywhere. Can I just use an emulator?

Given that two of the runs for the first category are emulated, I'd say yes.

I'm not a mod though, just someone who has this in their followed list in the case I ever do get around to it.

Yes! Feel free to use an emulator. Kega Fusion preferably, but whatever suits you.