This does not cause ambiguity to anyone. Its a distinction in search of a non-existent problem. The dictionary will change to follow our usage on this one, and we will lose nothing. Compound words become closed all the time.
This sort of word does not become compounded in the way "rain coat" can become "raincoat".
There isn't a single example where the verb form of any of these, as a single word, is considered correct:
backup carryout checkout comeback cutover cutoff failover handout knockout lookup lockdown login logoff logon logout meetup meltdown pickup rollback rollout rollover setup showoff shutdown shutoff signin signoff signout signup slowdown startout startup takedown workaround workout