rump roast was sired an baked. now to tough to eat . would simmering it in BBQ sauce to make sandwich's soften it ?
I don't think so. But you can still make pate, rillettes and alike... Here is a nice article that explains the physics of overcooked roasts and also gives some creative suggestions how to re-use the meat. http://www.bonappetit.com...
If it's possible, you might want to shred the meat. Then you could put it in the slow cooker with a ton of BBQ sauce. It won't cook the meat anymore, but it might make it slightly softer. Sorry about your roast.
Barbecue sauce, no matter how much you add, isn't going to do much to soften the roast. If anything it's just going to make your mistake less palatable. If you want to salvage it, braise the roast for an hour or two. Use stock and perhaps a half cup of wine to provide enough liquid to come about halfway up the side of the meat. Good luck.
I like Pierino's idea except I would drink the wine and throw out the roast.