I wonder if you are thinking of a "too perfect" secondary piping system. It wouldn't be subject to pressure so it would not have to be as carefully jointed as the pressure pipes. Indeed it may not even matter much if it leaked a little if the leak-points could be located where the leakage could be managed. Even a partial secondary system could be useful if it carried escaped water to a point where it would be easily detected. For example there could be several small sumps each with a small water level detector (perhaps just two bare wires that would conduct a small current if the water reached them.
The secondary piping would not even have to be a complete cylinder. Something like this would work
| O |
The "o" is the pressure pipe with a cover above and beside it that drains into a channel below. There is no need for the cover to be connected to the channel.
I suggest a compromise solution. Use square/rectangular plastic electrical trunking. The 'lid' would be strong enough to contain all but the most catastrophic of bursts under pressure and the availability of various sizes and fittings ('T's and elbows) would make it fairly easy to fit.
My idea for the water sensor is to use a piece of stripboard with each of the two wires connected to alternate strips, thus avoiding possible false alarms if the two wires accidently touch. Not a good idea if it happens when you're in the middle of a shower.