Here's a mod I've done a few times now, not least of all to my own guitars. I have done a Jazzmaster and a Mosrite so far for myself. When a customer contacted me asking for a "Local H guitar" I knew EXACTLY what he wanted.
So this is (was) a stock Ibanez Talman which are actually great guitars. The quality of the Japanese ones is great and even the later Korean made ones were superb.
The idea is simple. Put a pick up under the bass strings and send that to a separate output for bass and the rest of the guitar plays like normal through it's own output. I only run the "E" string through my rig but this customer wanted the E and A. If you're using an octave pedal, the trick is to get NO bleed on the D string. In a drop tuning you can play both strings through an octave and into a bass amp and the bottom two strings will always play the root notes. My guitars just having the E always plays the root note on any E bar/power chord.
Lining up the bass pickup is important so you just get what you want. I was trying to make as little impact on the guitar side as possible so a tilted P bass pickup between the two guitar pick ups was the best way to go.
Once marked out I routed a small chamber for the pickup to go into.I don't like to rout a "swimming pool" under the scratch plate so it was a snug fit.
With the pick up in the body it was just a matter of cutting the scratch plate to suit and then wiring the whole lot up.
The P bass pick up is easy to fit and adjust so the plate is just cut to the shape of the pickup and there's no holes to drill.
The two circuits are kept completely separate with a volume control for the bass pick up (in the original Talman vol pot) and then and guitar side is kept the same with the three way switch and the original Talman tone control converted to a push/pull volume pot. The push/pull controls the humbucker/single coil on the bridge pick up.
With everything back in and working it was just a straight set up. These Talmans play great and the necks are always so good on them. The set up is just a straight guitar set up. The guitar is played as standard but now has the option of sending an independent signal off to a bass amp.
It can of course also just be used as a standard guitar too and can be made back into a "normal" looking Talman with no hint except the extra output jack .