Can you handle her at all? Will she respond favorably to a scruff hold? How about a scruff hold and a towel wrapped over her front half?
She is going to need to have those stuck poops removed and shaved in a 'potty-patch' would be best. I have to shave a potty-patch on Squirrely-Jo, but I am able to do it at home.
Don't tell my husband, but his hair clippers work better than my horse clippers.
In 2007 I found a very badly emaciated feral cat on my back patio who had collapsed. Think: skin & bones, literally. I was able to bring him in to try to nurse him to health, but after 6wks of fabulous food and care, he was enjoying learning how to be pampered and loved, accepted into our household of cats (pushing a dozen at that time) he continued to lose weight so I knew he was at the end of whatever was wrong with him. His bloodwork indicated possible cancer, so I am glad I was able to pamper him for his last weeks. Anyhow, at his first visit, they gave him an injection that imobilized him and brought me a brush so I could groom the snarls out of his coat and they could examine him thoroughly, because he was *wild* and did try to fight. He bit one of the techs in the arm and that is when they decided to give him the shot.
It worked pretty quickly and allowed everyone to do what needed to be done. I do not remember if they gave him a reversal (I think they did, but I'm not sure) or just had to let it wear off on its' own. When I had decided to euth because he was continuing to go downhill, I asked for some sort of sedative and they gave me a pill to give him that did make him loopy. They instructed me to give the pill about 30min before our appointment and it began to work in short order; his eyes dilated and he became uncoordinated. Louie stayed next to him (Louie is like my nurse-cat) and helped him to remain calm and still until I loaded him into the carrier for the 5min drive to town. When we got there, he did 'wake up' a bit and got slightly combative at being restrained for the final shot and they had to put the towel over his head to help calm him.
So, ask about a pill or the IM shot. I'm sure either of those would be cheaper...and if you can handle the kitty at least 'okay', you may be able to clip the potty-patch yourself at home by using the pill. Put a comb between the poop-snarls and use scissors to cut the fur above the comb. This prevents you being able to pull skin into the path of the scissors. If you have clippers, lift the tail and give a quick shave. If you can't handle her, ask the vet about bringing her in and using the pill or the IM injection.
The pill seems to dope-them-up and the IM injection imobilized him for about 20min or so...