I haven't used them but am interested myself. They have an excellent reputation among recumbent and tandem riders. If you haven't located it already, here's their website http://www.sandsmachine.com/

And the framebuilders list shows 3 builders in Seattle. IIRC, they don't recommend retrofitting aluminum frames but it's ok to use the couplers if it's built with them from the beginning.

I wish these builders wouldn't do their own photography , but these are some fun custom carved lugs--Clavin and Hobbs, penguins jumping from a cliff, etc.
http://www.sandsmachine.com/carvlugs.htm