you need to sit through mud otherwise you are going to spin all over the place. if you are moving so slow you feel the need to stand to get through mud you should be running. its all about getting the proper line with the proper momentum to keep the pedals turning and the bike moving forward. once you feel yourself slowing down you need to get off the bike ASAP before you lose any momentum for a run.
in anycase even though that race was a mud fest last year its super rare conditions for this district. last season there was an even muddier race at the end of the season but it was loose thin mud and not that thick peanut butter ****.