It looks like NASA has flown payloads onboard the X-37B before, so I think it is well within possibility that they could get some room on board for Mars samples during a return. They might not even need to “book” a whole flight as long as the mission has room/capacity for the samples aboard, and the sample container could probably hang out in a medium orbit for a while after getting back to Earth, awaiting an X-37B mission to come up at its own leisure since the orbiter could do all of the rendezvous maneuvering on its own
It looks like NASA has flown payloads onboard the X-37B before, so I think it is well within possibility that they could get some room on board for Mars samples during a return. They might not even need to “book” a whole flight as long as the mission has room/capacity for the samples aboard, and the sample container could probably hang out in a medium orbit for a while after getting back to Earth, awaiting an X-37B mission to come up at its own leisure since the orbiter could do all of the rendezvous maneuvering on its own