Have a look in the tandem bike sub-forum. Lot's of people have solved this in lots of different ways to allow their stoker to have a bike computer.

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

Yes, they can, and some cyclocomputers are made to be mounted that way. I bought my vintage Trek 520 from a guy here in town, and it came with a Cateye Astrale (also a vintage piece) already installed; it has a magnet on the left crank with a sensor on the front part of the chainstay for measuring cadence, and a magnet on the inner rear wheel with sensor on the rear part of the chainstay for measuring speed. It, however, was made to be hooked up this way. Other cyclocomputers can be Jerry-rigged to similarly work on the rear wheel. I took a $10 Bell (-gasp- Wal-Mart brand!) cyclocomputer and routed the cable up over the toptube and down the chainstay about as far as it would go (it wasn't very far, but it was far enough to reach the spokes of the rear wheel), and I chipped the corner off a piece of foam insulation that just happened to have an elliptical edge that would be just perfect for fitting itself to the seatstay; this provided the extra space buffer I needed, since the magnet needed to be a lot closer to the spoke/sensor (it was made to be on the front wheel). Then I simply zip-tied the sensor and the small piece of insulation to the seatstay, put the magnet at the appropriate spot on a corresponding spoke, adjusted the distance between them, and voila, it worked! I actually used that setup all of last winter to keep my cycling fitness up to a bare minimum...I've found I ride a lot harder if I have something to tell me how hard I'm actually riding.

oops...sorry for helping resurrect this thread; I just realized it was three years old 8P

You have what is know as; "The Two Watch Problem". It goes; "The man with one watch knows what time it is. The man with two watches is never sure."

I have Cat-Eye Enduros on two road bikes and also have a Garmin Forerunner 305 GPS running/bicycling watch. I'm amazed on how closely the Garmin and the Cat-Eyes agree as they read within 0.1 mile of each other on 35 mile rides. However, I do set my Cat Eyes slightly lower than the recommended calibration number. For a 700x23 tire they recommend a calibration number of 210 and I use 209.