Thanks for the feedback.
MOD REWRITE
Mod rewrite can be server intensive, so I wanted to stay away from it.
UPGRADES
For people who want to install this and keep their x-cart upgrades manageable, I would just backup all the files you change now. When you upgrade, revert to the backed up files, do the upgrade, and then reapply this mod. Probably faster to reapply this mod than to spend time with .diff's
KEYWORDS
As mentioned, you can put keywords in the .html file name which can be anything.
If you wanted to increase key words, you can always modify this (as mentioned you can be as creative as you wish) to add a secondary folder, so that you have
http://www.domain.com/product/FAKE_KEYWORD_FOLDER/PRODUCTID/key_words_here_too.html