I have already addressed this question, by stating that I don't think any fighter (especially a Western one) would hide out in a very uncomfortable and inconvenient country with no infrastructure, just to avoid USADA.
However, if you had the mentality for it, I believe it is a loophole in the USADA system. In some countries in the world, you can't just enter and leave at the drop of a pin. Safe travel, accommodation, communication and security have to be organized way in advance.
So, if you are still dirty and USADA wants to test you. You can use a lot of reasonable excuses to delay the test, until you are clean.