Liam,
I don't think the drill guides are designed for hogging out mortises. Usually the largest bit they can handle is around 1", and are meant for lining up long holes for bolts or pegs.

I think you would be drilling twice as many holes as necessary and putting unnecessary wear and tear on a precision tool.

If you can't get a chain mortiser, why not a boring machine (hand-cranked)? Jim Rogers always has some for sale. Otherwise, many of us started with 1/2" electric drills that can handle big bits. For safety and control, I'd recommend a right angle model, like a Milwaukee Hole Hawg.