Even if they accepted the bag, I'm not sure I would want to check in the bag for the PRG flight in the morning.

Imagine the probelems that you would cause if the inbound MAN-LHR was delayed or cancelled, or you missed it. Not only would your bag get offloaded and go off to the bowels of the T5 luggage dungeon, but you would have some explaining to do, as to why you caused the PRG flight to be delayed.

I wouldn't do it myself. Take it to MAN and leave it, re-checking it in on the return, if you want to increase the chance of it going missing at LHR. If you actually want the bag for the weekend, then leave it at T5 left luggage and retrieve it and check it in on your return.