How to make a pie crust out of plantains?
This seems like a fun adventure. I'll say upfront that I've never tried what I'm about to suggest.

When I read your question title the first thing that came to mind is a Puerto Rican sandwich that is very popular in Chicago, the jibarito. It's a sandwich in which the bread has been replaced by flattened and fried plantains. I think doing something similar could yield nice results for a pie crust. You just press the peeled plantains between two cutting boards to get your desired thickness.

As far as not frying, you should also be aware that fried plantains are damn good. I've never eaten a plantain that wasn't fried. That said, if you are committed to not frying I have suggestions as well. 

First, the boiling and mashing is right out. I don't think the processing you'll be doing will result in a crust that holds up at all.

Roasting, or better yet, pan frying in butter I think is the way to go. I suggest just greasing a large frying pan with butter and pan-frying the plantain over medium-low heat until done. 

If pan-frying is not your thing either, I'd suggest just brushing them with melted butter and tossing in a 400 F (200 C) oven for 10-15 minutes (complete SWAG - never oven roasted plantain). With either the roasting or pan-frying method you may wish to sprinkle each buttered side with a little sugar to add a little sweetness and a nice carmelization.