how would I add integration for some of the dynamic storage crates (such as slotmap), without making it a dependency (only using the impl if the crate is already being used)?
You would need to make slotmap an optional dependency that would be enabled by whichever downstream crates require slotmap support in deepsize. Check out:
Moved from https://github.com/dtolnay/request-for-implementation/issues/22#issuecomment-457410613: