Goblins already supports distributed (acyclic) GC! And yes ocap security does tie in nicely. In a sense ocaps make this really easy... if you think about an ACL approach the user has to point at the object they have access to and the object has to point back, but since ocaps are based on "having references to objects", in general GC locally can just be the very GC that the native runtime provides. Across runtimes (in the distributed code), distributed GC likewise falls out easier, though the algorithm doesn't know how to handle cycles when Alice on machine A points at Bob on machine B which points at Alice on machine A again. So you have to manually cycle break in those cases, but they're relatively rare.
Amazingly, Electric Communities Habitat had distributed cyclic garbage collection, which just blows my mind: http://erights.org/history/original-e/dgc/
But that requires special cooperation to traverse the object graph held by the runtime's garbage collector, which we don't have access to in Racket (or most languages) unfortunately. It would really be cool to see that tech revived somewhere though.
Hope that answered your question somewhat!