Hey, Thank You for sharing Your knowledge and experiences! I like wild ideas

You can be sure that I'll make use of those connectors. I can't guarantee success of my approach, but I'll surely check what can be done. Even with FM TX - it's antenna connector seems surprisingly reachable.

As for backcover modification, it' already started - check my signature. I've already implemented fully functional camera lens cover, now I'm working on including standard tripod mount (the main problem is space limitation, cause I use self-made dual-scud battery), stand "leg", and other ideas

As for attaching digital compass to N900, one guy already successfully connected one via Arduino and bluetooth, but of course, i2c approach is much more sane.

For sure I won't attach tv-style antenna - i prefer modularity via connectors. The craziest idea - not requiring mugen cover - would be to attach every antenna test connector to jack, and use such plugs to attach every antenna - one existing port, to connect them all (but not at once ). Ho ever, with dedicated "bulkhead" connectors, there is more chance for less gain loss and more noise reduction - I prefer that, while also I don't main another connector in my (Mugen) backcover.

What wonder me now, is point, where the antenna connectors actually connect = do we need to apply own impedance correction - after external antenna, but before N900 internal connector -, or not, cause impedance correction is still ahead of connector. Even if so, our antenna will most likely have different impedance, so to not loose almost all gain, we may need to perform impedance matching, anyway. Maybe "they" already standardized expected cable impedance (it's logical, that micro connector will join antenna via cable), to 75 OHM. Worth checking and determining - if not available in schematics - via experimenting.

Thanks again for photos and interesting info, about "struggle"