Sorry to join in without a real answer but just an idea.
You can have multiple carts on 1 site if Im not mistaken so although it would take more work on your part could you not use the same product database using different columns for the pricing in different currencies then depending on which currency the customer picks depends on the pages and add to cart buttons etc that are shown.
Using a session variable to store which currency the customer has chosen so the show region behaviour would know which buttons etc to show and use.
Possibly someone from WA would verify if they thought this was possible or not.
It would be a little complex. Just a thought.