Hi Jim - Late on my reply, but I just wanted to get back to you to say your recommended fix for routing RigPi TX output to the Yaesu FT-450D RJ45 mic connection worked great.   Nice, processed voice signal, right in the middle/normal range for mic gain.   Thank you again for your help and circuit recommendation.   Greatly appreciated by my 93 year old Dad using my station remotely from his apartment 600 miles away.   With your permission, and of course crediting to you, I would like to recommend it to others in the same situation.  

Our MFJ friends market the Yaesu 6 pin data port cable as "the solution" for audio.  Sure, it works great in 1's and 0's digital modes, but its use effectively bypasses the SSB-critical audio signal processing (compression, equalization and gain) that is processed via mic input.  Looking at the FT-450D service manual,  the schematics confirm it. I wonder how many hams without USB audio ports have given up on RigPi SSB voice communication after being unable to get their audio right on raw data port connections.   MFJ should offer a mic to RigPi TX Input cable option as well.

Thanks/73,  Steve KE4LC