I figured I'd start a new thread to focus on design specifics and semi conversion challenges.
For now, I'm toying with a sheetmetal shell that will need considerable bending work to get right - needs the rails bent, the lower "dimple" for the feed pawl arm, and I don't have any internal rails or the "ratchet plate" for the charging handle. I may throw some money at coldsteelsolutions if I can't figure all these details and use one of his sheetmetal shells instead. Time vs. money.
I figure I may be able to fabricate my own "ratchet plate" out of some steel tubing heated up and bent outward to the shape I need. Otherwise I'm not sure how best to go about fabricating this part.
About the internals.
I found a drawing of that Sig rifle I was thinking of - the STGW.57 It has a "firing lever" that the hammer strikes against. *Exactly* what I was envisioning!
Note how the hammer - #14 - strikes against the firing lever - #11.
This was found at the swissrifles site: