This is hit and miss...
If you copy link location from amazon, you get the desired link syntax:
http://www.amazon.co.uk/exec/obidos/...Fencoding=UTF8
The rewrite works after the ASIN to add the affiliate code, so this works.
But, if you click on the link, the sytax in the address bar changes to:
http://www.amazon.co.uk/gp/product/0...lance&n=266239
So, if someone copy&paste a link of a product that they are currently looking at, it won't work due to the /gp/product syntax.
So, let's just hope everyone copies link location and pastes that so that the ASIN stays valid!