It's not practical. This is not a matter of replacing protein with protein, this is a matter of providing a balanced, healthy and natural diet for a redfoot which requires animal protein. Mushrooms contain vitamin D but I would not feed mushrooms in place of UVB exposure for the tortoise, the same principle applies here. Please do not withhold animal protein from your tortoise, they are not built like human beings and are more sensitive to their diet. You could make an attempt to feed him no animal protein whatsoever at the cost of his wellbeing or you could feed him a diet that we know is healthy and good for him.
I agree. I own a rescue and I have no shortage of respect for people that take in torts in need, I know how difficult it is to acclimate to the needs of a tortoise, especially with no experience with other reptiles and especially with a tortoise that is worse for wear and may need extra love to bounce back to health. RFs are more sensitive to diet and habitat than many other species, which makes messing around with diet a lot riskier. Too much or too little of one thing and you get diarrhea, too much or too little of another and you get constipation. Animal protein in my experience helps stabilize the digestive situation and is good for the RF, I don't think eliminating it from the diet is a good or safe idea.
