Your close but not quite there yet
I've already got such done, and is in working condition & tested on live sites, mod_skinswap:
http://www.cartlab.net/lab/customer/product.php?productid=27
New version is about to be released which includes some of the new features:
- Admin configurable
- Disable/Enabled Swapping
- What to swap with, Store Language, Affiliate ID, Skin variable or any other variable.
- Default skin (fallback) if none is specified
- Use a skin repository directory to store all skins, or use subdirs directly from the root of xcart/
- Set which debug template to use on "Enable debug console" in General Settings of admin. (debug.tpl, or debug_tempaltes.tpl)
- Cookie lifetime of 1 year of skin choice.
- Check if skin directory specified in swap variable exist if not fallback to default.
http://www.cartlab.net/images/skinswap_ss.gif
The new version will be either released today or tomorrow.
Kudos!