Did some tests last night and they worked! First time! This is an unusual occurence for me, as things usually fuck up straight away and I end up giving up in frustration. I am properly excited about it now though, and I can't wait to try it out in the shoe factory.
Hopefully you'll understand what I'm trying to do now, as I am aware my previous explanations were a bit rubbish. Just try and imagine this on a much larger scale, with way more shapes and more interesting things happening, without a kitchen table in the way...
My First Map from Retchy on Vimeo.
Edit - 01/03/08: It's been brought to my attention that I didn't actually explain how I did this in the end. So for those of you that are interested, here's how I've managed to do it without any programming trickery, which is what I feared may be involved.
The first thing I did was stumble across Deepvisual's tutorial on Youtube about how to do 3D mapping in Modul8 - here it is. Luckily, I have a copy of Modul8 and it was a great relief when I discovered you could do it this easily. To summarise his video, it basically involves taking 2D shapes into M8 and using the Perspective Transform Freeframe effect to distort the shape as if it were in 3D space. You do this as you are projecting onto whatever you want to map so you can see when it's lined up correctly.
Once you're happy with how it's lined up (and it can be a bit fiddly with this method), you need to render a frame of all of the shapes as they are set up in Modul8. If the map isn't quite right and leaks over the edges a bit, you may want to take the render into photoshop to trim the edges until it's right - this is completely trial and error and you'll need to keep importing new versions into M8 to test them as you go, but if you keep the current version projected onto the shapes you can make slightly better judgements by cross referencing. If you're doing more complex shapes than just triangles (like badly made cubes with curvy edges!) this can take quite a while.
So you've got the mask finalised, now you need to animate it. I use After Effects, so I took the still frame and used it as a template for my animations. Make sure the movie is at the same resolution as you have set up in M8 (in this case it was 640 x 480) so that it lines up properly when you bring the final animation back in to be projected. That should be it, as long as you've kept the projector in EXACTLY the same position the whole time. Pretty simple really if you've got all the right kit, but this is probably not the most efficient or accurate method, and perhaps wouldn't work for more organic, irregular shapes (although I haven't tried this). It works for my purposes at the moment though...hope this might help someone.