Electronic – arduino – Three UART connections to an ATMega328

Thanks for everyone's comments. I heard back from TI with the following answer regarding this issue.

"Due to something innate in the Mindtree stack, you cannot generate an interrupt in the MSP430 while data is being sent/received via BT. Therefore, you will see a limitation on the throughput in/out of the part. This is something that Mindtree was working on and would be good to ask them through your posting to the E2E forum since the Mindtree engineer “Balaji Jeyaraman” may be able to tell you if they’ve found a way around this or updated the stack. MSP430BT5190 + CC2560 data throughput/loading issues

Along the same lines, TI has posted a Wiki (different than the Bluetooth one) that focuses just on this ‘BT bridge application’. It can be found here: CC256x MT UART BRIDGE We have seen similar limitations and the data rate you are getting is a little faster than what we’ve seen. "

So not the answer I was looking for, but I thought I'd post it here for others. Thank you.

Single transistors don't really have any "quiescent current". If you don't feed them some current (or, in case of MOSFETs, voltage), they don't conduct. (A couple of nA leakage current can safely be ignored.)

As @geometrikal stated, these days there are better ICs for your application. Modern port expanders have an interrupt output, so you don't need the array of diodes which your solution would require. This particular IC has a standby current of just one µA, so you don't need to turn it off.