The context here is that we 'need' some way to run integration tests for non-Matrix-standard features, such as those implemented by custom Synapse modules or maybe even Synapse-specific behaviours (if/where there is good reason).

Without knowing too much, Complement seems basically perfect for this use case, but it really wouldn't make sense to add these kinds of tests straight to Complement's repository.

It would be nice to have out-of-repository test suites (for example: alongside a Synapse module, say https://github.com/matrix-org/synapse-user-restrictions/, but you can also imagine weirder customer-specific modules).
Is Complement suitable for being extended this way? What would it take?

I'm not familiar with Go, so I can't easily suggest the right approach (I can speculate, but I'll refrain from doing so for now).