Some initial thoughts on this:
- If a product allows multiple currencies, which is used when going through checkout?
- Do you intend to allow multiple items in the cart, all (potentially) using different currencies?
- Do you intend to support various pricing adjustments like catalog and/or shopping cart price rules?
- Do you intend to use other modules that make pricing adjustments?
Honestly, I see too many problems with this to be practical. Now, if your catalog only had a handful of SKUs and was fairly simplistic overall, this could work; however, considering all the additional features that can adjust the price, I think you are approaching this the wrong way.
I would maybe consider a different approach where you set up multi-store / multi-currency as one would normally. Then, on your product view templates (maybe category / other areas as well, depending on your needs) you add an additional block that will display the product's price for the other stores and make this price a link to the product's view page in the other store where a user can then add to cart and check out in the currency of their choice.
You might be better off sharing the purpose of this modification. I get the feeling the PM on is creating unrealistic expectations, and that things have been promised to the client that are not possible or much too difficult to remain practically possible.