You can do your own pinning, which is what I do in gobind[1]. The basic idea is to keep a map in Go of the pointers:

    var ptrs = make(map[uintptr]unsafe.Pointer)

When passing a pointer from Go to C:

    var p unsafe.Pointer = ...
    id := uintptr(p)
    ptrs[id] = p
    C.Fn(id)

When C returns the pointer to Go to be used, run it through the ptrs map again

    //export GoFn
    func GoFn(id uintptr) {
        p := ptrs[id]
        // ... use p
    }

Don't forget a cleanup function where you delete(ptrs, id). If you want to pass the same pointer multiple times to C and keep it comparable, you'll need a second map.

All of this is requires being very careful, but the hard part is the notion of holding references to memory outside the realm of the GC. I don't believe a runtime-assisted pinning API can do any better than what you can do yourself with a map.

[1] https://godoc.org/code.google.com/p/go.mobile/cmd/gobind