I was able to pick up a cheap guitar from a PCV who was leaving the country while I was training. It was MUCH better for me than bringing my guitar from the states. However, I am guessing it will be much more difficult to find a PCV leaving behind a banjo. It doesn't hurt to ask, though. While I didn't have tons of time to play during training, I found the time I did play it was a great way to relieve stress. For that reason, I would not necessarily recommend waiting until you are in country unless you are sure you will be able to find one. You may consider shipping your banjo ahead of you to a currently serving volunteer or the PC office (If they allow it).
I believe 2-3 volunteers in my group brought their guitars with them- soft pack on the plane instead of a carry on (I think, not sure if they paid extra baggage fees for them).
The only advice I have is that if you are carrying an instrument, make sure you pack light with other stuff. Carrying your belongings to your training site, through airports and local transportation, until you eventually get your placement can be exhausting if you have too much stuff. But a quality instrument is IMPORTANT for volunteers that play- it is a great way to connect with host nationals and can help side projects. So getting down to one suitcase and your banjo packed in a hard case for checked luggage or a single checked and a soft case carry on would be ideal.