Hey Dr. K!
I hope that all is well. My primary splitter is a Boss TU-2. It's effective and cheap and it can be modified. When I have to tune, I can cut the signal to the FOH or when I want to work on my stage sound or just practice without hearing my gnarly tones crush the FOH drivers.
Any volume pedal with a tuner out can be modified to perform such a task.
As far as all these guys doing these crazy mods, I have this to say... I hope that it's fun because it sure sounds like a lot of work and expense. My personal suggestion (take it or leave it because opinions are like...) is to tune the rig that you have in as many different environments as possible, using limited or no EQ on your bass. That way, your signal to the FOH is consistent. When I started out, I was broke and HAD to make what I had work so that I could eat. I forced myself to not change the bass and use my hands to change the sound. Eventually, meeting Fender and A-Designs, I was able to attain the variety that I needed.
Learn how to create as many sonic options as possible with what you have. After that, it's easier to make an informed decision as to what to buy.
I always remind myself that Larry Graham and Jaco Pastorius played nearly identical basses and rigs so it ain't the shoes.
Thanks for letting me rant, Dr. K!