Has anyone ever heard of substituting mashed beans for fat in baking?  Someone told me that they had chocolate chip cookies made this way that were very very good, but they could never come up with the recipe. 

I haven't heard of substituting mashed beans for fat.  I have a couple of quick bread recipes that substitute the fat by using apple sauce.  I also know that you can substitute flax seed meal for part or all of your fat, but it depends on the recipe.  I'll keep my eyes open for the mashed beans as a fat substitute.

I think apple sauce would be a better choice.

 Never heard that one. I've used applesauce and yogurt as replacements.

I have used applesauce , canned pumpkin, baby food plums. 

I found this link about substituting beans for fat. Interesting.

http://linkinghub.elsevier.com/retrieve/pii/S0002822305006358

 Sure hope it helps