I routinely use a Nikon DR3 angle finder with a range of lenses, often in dark woods, and I find focussing is no more of a problem then when viewing directly, so light loss should be minimal. The DR3 does not magnify the image. However, you don't really want a right angle finder, and the prism will degrade he image a bit, and add to the price.
More suitable would be the Nikon DG-2 eyepice magnifier, which can flip out of the way for normal viewing, but I doubt it fits Canon.
Fortunately Canon have an equivalent called the Magnifier S, but sadly it has been discontinued:
http://www.majid.info/mylos/weblog/2003/07/06-1.html
I'm sure you could find a second hand one. I bought my used DR3 on ebay for £24 compared to ~£100 for a used one on the high street. So with luck you will get a Canon Magnifier S for not too many readies.
As an aside, I wrap bright yellow tape around my DR3, so that when I put it down on the ground, I will find it easily. I lost my first DR3 in the New Forest. It was in mint condition too.
Leif