Ah, I see. I hope there is such a user as Rick60! If not, then I shall have to seriously consider my state of mental health. Ha!
Personally, I would not use Oxypowder while taking slippery elm gruel. I would not warn against it, though, so you can certainly self-experiment; however, I would suggest making sure you take the Oxypowder as far away from the gruel as is possible. The gruel is a food after all, and must be given time to do its great work. If you are following a liquid diet then you must make sure you are experiencing at least one bowel movement per day, so perhaps taking six or so Oxypowder capsules last thing at night may be the wisest choice. (I believe this is the standard dosage?)