This is an interesting idea. I've seen similar on Ebay. I'm guessing it could be done without too much trouble with modification to the code. I'd start by searching for where the xproductsperpage configuration is used. I assume that is a compare statement there that could be extended using a variable that would be set using the "View All" button. It seems like the tricky thing would be clearing the variable.
....okay, I looked into this a bit for you and found that it is handled in shopdisplayproducts.asp in this line (aprox line 141 in ver 5.50):
ShopOpenRecordSet SQL,objRS1, mypagesize, mypage
mypagesize is the key here: it holds the value from the xproductsperpage config.
Unfortunately, I don't know enough SQL to do anything w/ this at the moment. I have some ideas, but I'd have to experiment to see the result. I'm sure someone else on here knows this well and can give solid info on what to do here.