Hmm.
How thick is the foil? If it's thicker than a few dozen microns it might not etch well.
How would you stick it to the board? (edit)Oh, the tape is adhesive I guess right?(/edit)
You'd need to etch it *after* sticking it to the board and you'd need to be sure whatever you used to stick it doesn't dissolve in your etchant and is free from bubbles or any type of gaps, otherwise you'd end up etching underneath, on the non printed side... It would also have to keep the foil stuck firmly in place while sanding off the toner and soldering everything.
By the way, the fuser module in a laser printer (required for etching since laser toner stands up to etchant) gets to very high temperatures. Putting wax paper through a laser printer = fire.
If I were etching my own circuit boards I'd either use the toner transfer process (which I have my doubts about, yet tons of people say works great (rckstr uses this method iirc)) or if I wanted to be ghetto about it I'd draw the circuit in with an etch resistant pen (whiteout works for this) or I'd cover the board in tape and cut the circuit out with an exacto knife.
Copper clad board is cheap as dirt,
here's a chunk the size of a sheet of paper for $3.35.
Neat idea though, that instructable opened my eyes to the world of "Pyralux" and gave me renewed hope for replacing flexible ribbon cables in laptops I accidentally rip on occasion, which is completely devastating by the way since it's usually within the LCD and is normally about $200 to replace the whole panel.