I don't think the complexity would be that great. It's not like different tax rules apply, it's just a different numeraire. Now, remember, people reporting their taxes are not forced to change anything, they are just given the opportunity.

I guess the basic reason I promote currency competition is that I think people have the right no use whichever money they prefer. And in order to not give preference to the national currency, produced by the central bank, it must be possible to report ones taxes in another currency/money.