The BT50 claims 50m distance (open) but with a 8 hours battery life, which will shrink with time. You'd need longer battery life I guess?
I think the software would be easy.... if you know what you are doing. That might be the plumber joke of knowing where to whack the pipe. I "feel" like it wouldn't be hard.
The WT9011 should be able to send sufficient data. Practical in that it has iOS SDK, so very familiar territory for the author. However the battery life is very bad at 8 hours, the cost of transmitting continuously the raw samples... It would be a rather simple hack to increase that by 3-4x, at the expense of added bulk. As it is a standard LiPo cell inside. Device itself is very small, and has charger integrated. I did a quick mechanical review of this device last week. Have not checked the software yet.
Very useful links! In the end, seems like my best option is to use a BLE sensor of some sort (others mentioned vibration sensors which also look interesting). Thanks a lot for your help!