To get it out of the way: It is not possible to have multiple currencies in the manner your proposing on a single store at this time. For large inventories the normal option is having multiple stores , at a discount plan rate , for internationalization and region control. Base currency carries through to checkout and must be so because of the number of payment gateways shopify supports out of the box.
There are 3 separate things here, actual price, displayed price, and converted price.
Actual price: is set in the admin, stores can only have 1 currency in this regard.
Displayed price: it can be anything you want in regard to currency, it can be discounted, marked up, or converted.
Converted Price: can be just a form of display price.
BUT the catch is only Actual price is displayed in checkout
Q:So what to do? A: Just set Actual price to regional-price
This means inventory mgmt overhead is the tradeoff. So per regional-product(or variant) has it's price adjust and is organized by an as yet undefined convention to assign anyone from a certain geo-IP the correct region-priced product line.
The complications here are:
- If product actual-prices need to be set frequently by exchange rates
- Showing the right products on the client-side
- Validation when orders are submitted.
- geo-IP is not 100% reliable.
- Customer dissonance they must be informed that checkout prices will be displayed in the stores default currency but adjusted to currency-X
Depending on shipping requirements you may also want to make a custom carrier service via the api.