If you do a search on other messages in this forum regarding MIDI, there was another thread on something similar to what you may be wanting to achieve.

I had suggested in this thread that with some code in the serial interrupt, you could pass through the MIDI messages you weren't interested in and to trap and modify the messages you wanted. This used a similar method to your test code in that you echo the rx bytes out the tx rather than buffering them. This had the advantage of creating a minimal delay.