Actually, I decided to use the comment style shortcodes before the official shortcodes existed. Adding proper shortcodes is on my long list of things to do sometime, but I won't remove the comment style ones because that would break exisitng sites. Anyway, I'm glad that the rendering order setting solved the problem.
"what would it take to allow the user to select a currency to override the one distilled from their IP address, perhaps by way of a dropdown widget?"
They should already be able to do that - there should be a drop down list of all the currencies at the bottom of the post. That's needed in case the IP lookup is wrong or if they are an Australian living in China who wants to see it in Australian dollars (as I was when I first wrote the plugin). :)
Overriding the default currency is a great idea (and one that I think I've heard before). I'm not sure what's involved in do that, but it shouldn't be too hard. I'll have a look into it over the next week.
"Just tossed this into localcurrency.php at line 170 to kill the whole thing if the "to" and "from" currencies are the same"
I have very little time to spend on my free plugins these days. That said, out of all of my free plugins, this is the one that I think is most useful and which I want to keep going. I'm happy to keep improving it, although I may be a little slow at times. It's probably due for a complete overhaul at some point in the near future to make sure it's using WordPress best practices..