[sam0x17]

Totally understand about the B2B thing,

Regarding BLE -- is that really a limitation though? Even if the hardware / API won't give you a good indication of signal strength, surely there is nothing stopping you from taking a timestamp, sending a packet to the BT device, taking a second timestamp when the response arrives from the device, and using t2 - t1 to measure "strength"? There are papers on how people have been able to triangulate a physical location based on three ping times -- this seems a lot simpler as it's the 1-D version of that problem. Worst case scenario you need a nano-second precision clock, but the principle should still work.

[sam0x17]

Clarification: researchers have been able to triangulate a physical location based on ping times to 3 widely-spread locations.

[bradknowles]

And you can do the same with signal strengths.

Basically, anything that varies with distance can be made to work.