No way. and dh wouldn't want to either. Particularly if his mum was physically able to do it, but just didn't want to.
Would just say that it's ok, she can do it once she is back at home so that she is able to use and mess up her own toilet, and then she won't have the worry of having exploding projectile poo if the suppository (or if you persuade her to drink movicol, prune juice etc) works a little too well...
Because I am betting that if she is the sort of person that doesn't deign to stick her suppository in then she isn't going to expect to clear up any resulting mess... And because of that she isn't going to care if she has an accident or doesn't make it to the loo in time, because she knows she won't have to clear it up or live with any stains or horrors left behind.
Sell it as an advantage - she won't have to worry about dashing to the loo until she returns home (assuming she is not coming to you for too long, obviously this is not going to work as a tactic if she is planning on coming to stay until she has unblocked herself completely out of her own home so she doesn't need to deal with it for a few days.
Or why not change her invite - tell her to come once she has got rid of her constipation, she obviously won't want to be away from the comforts of her own home, in discomfort etc. Much better for her to deal with it at home and then come to see you when she is all better 
. This is what I would be (dare I say it) pushing for, but I guess it depends how much you like having your mil there!
Also do you have any dc that you can use as an excuse for not doing it - no, not that they do it - but that you don't think that it is a good idea for her to be around and needing to dash to the toilet when little dc is still at the stage of needing to rush to the loo when he remembers to go or because there isn't enough loo facility in the house to have her blocking one up for a long time... clutching at straws here to get you some decent excuses
Thing is, mil has already crossed a line just by asking you to do this! At least you can keep throwing it back to your dh if he says he's not going to do it by saying 'nor am I. She's not my mother - she's your mother so it's something you need to sort out together. Either you do it or you organise the district nurse (and get told not to waste their time) or tell her to come another time when she is better'.
Good luck - you have to let us know the outcome!