It's been done for years on consumer programmes though, and (importantly) for the same purpose, namely to expose dishonesty. I'm not sure I can see any reason to allow them to do it but not this guy.
I think it's a whole different avenue of journalism, reserved for Panorama/Dispatches specials and such, as you allude to. However, I don't think it's particularly becoming of one of Channel 4's senior news correspondents.