Can I ask how your shop is set up, and with which component? e.g. VirtueMart?

Is it possible in that component for *anything* to be able to change the display of the currency? e.g. is there already a mechanism for setting up an alternative pricing structure, and the possibility of changing to the alternative, e.g. via component preferences or individual user preferences?

The simple answer is that MetaMod can generally just decide whether to display a particular module or not, depending on things like GeoIP. This is probably not the same as switching your pricing system from one type to another, but again that depends on the capabilities of your particular cart system.

Sorry I can't help more than that without more info about your setup.