I'd say Qi is far more prevalent today that Touchstone. You can find Qi based items all over the place. In fact, there are apparently 3 separate charging systems out in the wild, though until to day I'd only heard of one. Guess which one is winning... Short of Apple grabbing one of the lesser specs (or inventing their own) Qi seems to have the foothold.

Most devices have Qi built in, or (if smart) have connectors under the back cover, and a slim insert or cheap back-plate with the receiver. That would allow them to simple replace that insert or back with another standard receiver if the tables get turned somehow. Right now though, that market looks to be 100% Qi based. (Samsumg, LG, Qualcomm and Nokia are all behind it... so there's a reason it's winning.)

The "best" way to do this with the N900 as is today would probably be a pass-through system. The power on the receiver would switch the external USB pins off (via relay or transistor) and connect on in a way that make it look like a charger of proper amperage. That allows the device to continue to know about charging in the way it's used to, eliminates port wear, and doesn't require a software re-write.

Doing that in form factor for the N900 would be tough. But if the Neo900 were to have that built-in with exposed pins for power-in from such a plate? May be a last minute request to throw in there?