While I haven't given up on the idea of converting a camera for infra-red, it turns out that my main SLR, a Nikon D50, is actually moderately sensitive to infra-red. The test is simple - set the camera to manual and an exposure of a second or so, then point a video remote at the lens and hold a button down while the shutter is open. If you see a bright light at the end of the remote it's sensitive to infra-red, just how sensitive depends on the camera. The older the digital camera, the more likely this is to work.

So I should be able to take infra-red photos without actually modifying the camera if I use a filter to cut out the visible light. The snags, of course, are that exposures are long - a second or two - and you need the visible light to focus the camera, so this is very much a camera on tripod and manual operation scenario, with a lot of trial and error. But I'm going to give it a try - I've ordered a cheap infra-red filter on eBay, if I can get it to work and get interesting results I'll look at a better setup.