Yes, a '72 OT barrel will fit on the R-M, and an R-M barrel will fit on a '72 OT. That is the '51 R-M, I mean. (some fitting required, of course). The '60 R-M is also made in .38 but I'm not sure if a '60 R-M barrel will fit on the OT or not. If ejecting brass is your only concern, you know that the MWNN has a two-piece rammer that easily comes apart to provide a brass ejector tool. Also, if you put a '51 R-M barrel on the OT frame, you will have no rear sight (which may be fine with you), and you will have what looks a lot like a '51 R-M. In that case you could save a lot of money by just buying the R-M, and you can get an aftermarket snake installed on the grip if you wish.
Update: upon re-reading your post, I'm thinking you already have a '72 OT. So yeah, you could add the '51 barrel to it if you wished (I know a guy who did that - looked cool) but you would have no rear sight unless you dovetail one in.