We are in the same boat, pigs up to their knees in mud, strawing them up every day at the moment and between them eating it or wiping their feet in it (they do love a straw doormat) my 13 were losing condition daily.
I was actually out on xmas day finishing off electric fencing so I could move them into a fresh bit of pasture, got the trough and water supplies sorted and moved them 2 days ago onto fresh grass. They are as happy as the proverbial now.
The lesson we have learned is certainly next year we may not over winter pigs at all, if we do it's going to be a smaller amount. We now have 3 or 4 areas we can move them between so it should be better for them but of course all the fencing has come at a considerable cost to enable us to do that.
I too was worried about the condition of an older sow but she seems to have rallied in the last 2 days as she's now on fresh grass and not up to her knees in soupy mud! I was even praying for a bit of frost which has happily arrived today, to firm things up a bit but as they always break through the surface and sink a bit then the frost needs to be good and hard to firm up the quagmire the top pen has turned into.
Keep at it with the straw, get plenty of food into them and you shouldn't loose too much condition. Stick at it!