I needed this for exactly one image, so now there's a Muttley LoRA. It's quick and dirty, so don't expect too much from it - in particular, Muttley's signature expression ("snickering") is harder to get than I'd like, and it's probably what you want. So you'll just have to describe it in more detail, "snickering, clenched teeth, grin, closed eyes" seems to work.

His Wacky Races look is "collar", while his Flying Machines look is "hat, scarf, goggles". Specifying "dog" can help the look, but you may also have to specify "no humans" then, otherwise you will get two Muttleys.

Curiously, this seems to work better with anime checkpoints than with toon checkpoints, but maybe I just don't have enough experience with the latter. As I said, it's quick and dirty, so maybe the toon crew should have a go if it's not good enough. Make a Dick Dastardly while you're at it.