Yes, you can mount a cycle computer's sensor and magnet at the rear wheel and traingle. I have it set up that way on my Vitus Carbone 7:



See the magnet on the spoke on the rear wheel at the 10:30 position in the pic above? The magnet sensor for the VDO wireless computer is mounted on the left, non-drive side chainstay, just a few inches aft of the bottom bracket You can just see a a small black box hanging down from the left chainstay in the pic. I now also have a cadence sensor mounted next to it with a magnet on the left side crankarm too. Both had been working flawlessly since I mounted the computer on the bike early this year.
Being a French classic bike fan, I take offense to your friend laughing at your vintage/classic Peugeot. Please smack him on the back of the head for us C&V fans the next time you see him, and also please realize that just because the bike is old, it does not mean you have to "put it to pasture" to a life on a trainer. Some of the best riding bikes out there are 20 years and older. Keep her on the road and just ignore dolts that think C&V bikes are funny.

Chombi