Too complex, and perhaps can cause some problems. I would just convert the price in PayPal module.
LOL, I must be doing something right if Andrei thinks my modifications are 'too complex' :-)
Converting the price in the PayPal module works as expected. The extra coding for always converting prices to the Working Currency (depending, of course, on a configuration setting) in the PriceHelper.FormatPrice() method is only for display purposes. I haven't found anywhere where the output from FormatPrice() is used in calculations (because, indeed, it shouldn't be) so I'm fairly confident that my changes will not cause problems elsewhere.
Nevertheless, I'm preparing a large batch of 'humble pie' should I be shown to have been overly optimistic.