Homebrew PCBs

I wouldn't really use wire wool unless you can get stainless steel 
because of possible rusting problems with ordinary steel wire wool 
which tends to rust in a few hours, smaller peices which can be 
embedded into the copper will rust very quickly.
As for the board warping, true, the copper will expand differently to 
the substrate but the amount of warping may be relatively large but the 
actual amount will typically be around tenths of a mm unless your board 
is *really* curling up;-)
Do you wait for the board to cool completely before soaking the paper?
Also, can you physically 'feel' the toner on the paper like braile?
I noticed this the first time I printed on the photo paper with the 
printer of 'transparency' setting.

PK