As others have said, good addition to the Wiki.
With regards to the legality, I too believe it is unlikely to be illegal. It brings to mind cable spiking (where to ensure that an electrical cable in the ground is not live, a steel spike is fired through it with something like a shotgun cartridge). If I suggested this - hey I know, let's shoot a steel spike through the 33kV cable to check that it is dead - I'm sure people on here would tell me it's too dangerous, illegal etc. and yet it is done.
Besides it is not illegal to own or transport hilti caps, so short of a copper being down every dig, there is not going to be any evidence.
And I have to say, NigR, you seem to be pretty keen on free and open access to some things, why not information? Especially where public knowledge may lead to improved safety, better techniques, etc.?
As for the tabloids, there are far too few cavers for them to care other than as a freak accident story. They are certainly not going to start a campaign to stop capping, let's face it, most tabloids are anti "nanny state". If I want to shoot myself in the bollocks, then one tabloid or another would probably happily provide the means for a good story. Of course the other tabloids would be against it for a day, until they got their own bollock shooting volunteer.