I'm not sure I understand the problem.
I have considerable experience with muliple currencies. The first thing you have to commit to is actually accepting the currencies. If you don't want to accept a currency, then don't offer it to the customer. But what is the problem with accepting the currency?? Does you bank not allow it? Everything is kept neat and tidy within ZenCart... it records the exchange rate used at the time of purchase, so even for audits all the information is still there. And all reports are done in your store (default) currency (all conversions are made). When you look at the orders, you see the currency the customer chose - but that makes sense since that is how they are paying.
The only significant problem with accepting foreign currency is volotility... if I accept a US$100 order and they chose to pay in Mexican Pesos, for example, I might tell them to pay 900 pesos, right? But then by the time I receive and deposit that 900 peso, it might only be worth US$80. For that reason, you should probably stay away from volatile currencies (Mexico, Russia, etc) and stick to currencies that you are familiar with and can do business in. Most banks anywhere in the world will deal in US dollars, GBP, and Euro and most likely Japanese Yen. Many also deal in Canadian dollars and non-euro European currencies without a problem.
I haven't released it as a contrib yet, but on my site I build in a "penalty" for foreign customers. The reason is that the conversion sites that zencart uses (xe and oanda) convert using "interbank" rates... the rates you might get if you are trading in the billions of units. The rate the bank actually gives you is usually 4 to 5 percent worse than that. So I have it set up so whatever rate the xe/oanda sites returns for a given currency, are increased by 5% before being displayed.
eg: Item is US$100 Canadian currency exchange is 1.1. Default, customer will see C$110. With my system they see C$115.50 (1.1 X 1.05). Most customers never even notice, but some complain that its a higher rate then they see on Google. I just explain it to them and welcome them to instead pay in my default currency and let them get charged the same penalty (if not more) by their bank or credit card company. IF YOU DONT adjust the rates like I do, you get screwed by at least 4% every time you accept a foreign transaction!!
So there are downsides and complications, sure. But if you have international customers, you WILL increase sales by allowing them to pay in their preferred currency. With that said... don't go nuts! Stick to USD/CAD/EUR/GBP/YEN for the most part.... plus, of course, whatever your local currency is.