During its run on the CW, 'Reaper' never gave any reason for it to be banished to an alternate TV dimension; even the appearance of the Devil differing from his appearance in 'Brimstone', 'The Twilight Zone', 'Eastwick' and other shows can be splained away by His Maleficence's supernatural powers.
But that doesn't mean 'Reaper' can't exist in other TV dimensions as well as in the main Toobworld. We've seen so many TV characters who had doppelgangers in the "evil mirror universe", for example. So it's pozz'ble, jus' pozz'ble, that the characters and stories of 'Reaper' could also exist in the world of 'The West Wing', where 'Castle' also exists.
But they don't have to have the same outcomes as in the original episodes. And with 'Reaper' cancelled, the fates of all the characters could conceivably go off in all sorts of directions, no matter which dimension they're in... as we may have seen in this week's episode of 'Castle'. In 'The Third Man', Rick Gonzalez guest-starred as Mickey Carlson, the first suspect in Detective Beckett's murder investigation. He was homeless and working as a delivery boy for the New York Ledger, and he made a habit of crashing in people's apartments when they went away on vacation.
It could be argued (and you know Toobworld Central will!) that Mickey Carlson was in fact Benjy Gonzalez, the character Rick Gonzalez played on 'Reaper'. Who knows what transpired after that show - even if this is the alternate version - went off the air back in May of 2009? It could be things went southland in the relationship of this version of Benjy and Nina soon after the "exorcism", and Benjy may have felt it necessary to get out of Seattle and live somewhere else for awhile. So he changed his name to Mickey Carlson and disappeared into the homeless ranks in the Big Apple. There's a pozz'bility that 'Castle' might be able to rejoin the main Toobworld due to a major reboot of the Toobworld timeline, caused by shows like 'Primeval' or 'Lost', or by smaller events never to be seen in 'Doctor Who', 'Journeyman', 'Quantum Leap', or even 'The Time Tunnel'. If so, this theory could still work if we're dealing with the Benjy Gonzalez/Mickey Carlson of the main Toobworld. And in the unlikely case that there's ever a reunion TV movie for the series to give it the closure it needs, this theory would all be in the past with no need to be mentioned - at some point before we revisited the 'Reaper' characters again, with Benjy reunited with his friends Sam, Andi, and Sock, and perhaps even Nina. Woiks for me!