> are there any GC languages that have a way to specify if something should go on the stack or the heap?

I think only in the sense that some GCd language have value (stack) types as a separate hierarchy from heap types? E.g. structs in C# or Swift are stack-allocated (and value-identity, and copied) whereas classes are heap-allocated.

Adding that for java is one of the goals of Project Valhalla I believe.