I've read that some folks have outstanding results with one o' them household steam cleaners like the "Steam Buggy" - it's God's Own Eraser for Cosmolene, and may well work wonders on old skanky grease. Retire to quiet place - remove grips - hold article of affection over 5 gallon pail using heat resistant glove - enthusiastically apply steam until desired affect achieved - throw away bucket. Been meaning to buy a milsurp just to try this out.
The trick, I suppose, would be to bake the stuff you've steam cleaned in the oven to rid it of any residual moisture. Wouldn't want 'er any more rusty. Probably a good idea to stip it down and do the parts individually as well. And for God's sake, unload it before it sees the inside of the oven.
The rust issue may be well dealt with by a product from C96 - "Gun restorer", I think it's called. There was an article in Guns magazine last month where the guy said it'll lift rust but leave bluing. Dunno about you, but that sounds like the horniest thing ever to me.
Cheers, and good luck,
Dan