I love homemade remedies! The lemons for elbows (and other rough spots) really should be with the actual lemon included, not just the rinds. The lemon exfoliates and lightens. However, if you only have half rinds leftover, they make perfect cups for the elbows. Nearly any mixture would work well, from a simple baking soda and olive oil paste (I also like almond oil) to an exfoliating scrub (plain old table Sugar makes a great scrub!).
For feet, perhaps the lemon cups can hold the heels, but not much else. I would use a full lemon half for that, but still prefer my scrubs and a good old pumice stone. The least you will get is a glow and a great smell.
Also search on google for keywords lemons + elbows or lemons + beauty. There are tons of great uses for our leftovers =)