I am so NOT an expert on human nutrition, but I can't see how you can get enough protein on only 800 calories a day. I only got banded two weeks ago, but I have lost 23 lbs just from the presurgery fast to now. I was told to try to get 72 grams of protein daily. I have been able to do that with use of Unjury protein powder in milk. It comes to about 200 calories per serving, and 4 a day gives you 80 grams of protein (20g per 8 oz serving). I actually LIKE the stuff, especially the chocolate. Now that I am starting pureed foods, I probably will cut that to 3 a day, and add some pureed meat as suggested by my doctor. I have been getting some of my fluids from V8 juice as well.
I understand not wanting to lose muscle mass. I'm rather proud of my physical strength. Before I blew out my knees, I could out lift many guys. I still have my "guns" (biceps) because I have to use crutches to get around, and I can still curl 20 pounders with my sons rep for rep. Just can't walk. (Sucks, huh?) Don't worry aobut offending me. I am past the whole self pity/depression thing, and accept the fact I'm a cripple. Wasn't an easy road, but I got through it.
So stick with high protein fluids and mushy-fied lean meats for your first line of nutrition, and you should be OK.