Mechanical (non-spandex) stretch fabric for use in MYOG Stretch-Paramo knockoff?

>Basically I'm trying to build a waterproof version of a Polartec Powershield jacket or similar. Close-fit, durable, good feel, wind resistant, breathable, but Waterproof to boot.

I went through a similar thought process last year. Infact I even PMed you some of my thoughts.

Once I realized that I didnt "need" stretch, supplex makes a lot of sense. It is light, tough, very breathable, wind resistant, comfortable next to skin, not noisy or crinkly. It is proven to be very versatile in wide range of conditions – read some of Roger Caffin's reports.

For cold/wet conditions you can simply use silnylon parka.
http://www.backpackinglight.com/cgi-bin/backpackinglight/myog_mntn_poncho.html

seriously, you cant beat this. what I like is that it is minimalist/practical/functional and not based on marketing or expensive, hi-tech fabrics. Its is something you can make yourself.

Josh, do you know if the pump liner is stretchy? If it isn't then a stretchy face fabric wouldn't be much benefit.

Better late than never:
Cioch offers kids Paramo pants and jackets, perfect for what you are describing, but they are not stretch. Paramo is SUPER supple though so I find it very good for active use.

The pump liner is not stretchy, but it is supple, so you could oversize it a bit and then cover it with a stretch shell sewn together in key places.