A small (2x2?) walk-in building with no input/output, only signal connections, with a reasonably large area inside to place complex circuit layouts in a smaller footprint.

Not gonna happen. Circuit connections are insanely bad for your UPS, you'll run into UPS issues immediately.

Yep, am aware of signal api limitations, I suppose a player would be tempted to use alot of them due to the small size ... hopefully Factorio improves the signal Lua API and this might make more sense

I implemented it for myself just for fun to try it out ...
https://www.dropbox.com/s/3rhy4nr0yv09wda/Screenshot%202017-10-08%2023.05.56.png?dl=0

One idea if this is ever reconsidered is a limitation of 1 input and 1 output signal per "mainframe" connection (4 connections) ex: 2 inputs, 2 outputs ... which would keep the lag around 4ms per circuit update per mainframe (with binary encoding, a single signal can represent 8+ status values, just like a real computer protocol).

Might make more sense as a separate mod in any case since it is solving a different space problem, I'll ping you if I end up doing that and using some of your code.

Nice work!

Is it a continuous cost, or a cost per-update? For driving recursive blueprints, where the update rate is slow (you're almost certainly waiting for construction robots to work their magic), having a walk-in integrated circuit could possibly be worth the performance hit.