Hi - to send a feed directly into M8 you will need to use Firewire; but you knew that.
If your switcher is something like this:
http://www.keene.co.uk/multi.php?mycode=BT942 it should work ok, there is a bit of a clunk as you switch, make sure you 'lock' the source on your projector so it doesn't get a chance to go off hunting for a signal, shouldn't happen.
If you stay on the video capture layer in M8 as you switch you will notice a slight glitch, if you don't like that fade to another layer then back again.
You will then need to convert the signal coming out of your switcher into Firewire with something like this:
http://www.canopus.com/products/ADVC55/index.php
Take that into your computers firewire hub or straight in, hey presto.
I've not heard of a software switcher that will do this. Let me know if there is one.
This is all a lot smoother & easier if you have a video mixer.
Hope I understood your thinking - Hope that helps.