The easiest way to do that would be to run two seperate coupling caps off of each half of the phase inverter. One cap on each side would go to your 220K grid resistor to ground (cathode biased side) and the other cap would go to a different set of 220K grid resistors that have a junction at the existing bias circuit.
The 2nd coupling cap would block the DC bias from getting to the power tube grids on the cathode biased side.
This does require some comprimises & requires you to make some decisions & tweaks based on listening to the amp. By splitting each phase inverter output 2 ways, you are going to be halving the load each half of the phase inverter sees. Instead of each side of the phase inverter running into a single 220K resistor, it is now running into a pair of 220K resistors that are essentially in parallel (110k ohms). Have you ever changed the power tube grid resistors in a Fender amp to 110K? Does it make much difference on the sound? Well, what we do know is that you will change the LF roll-off frequency somewhat but not drastically. Loading down the output of the phase inverter with half the value of resistor usually does sound different though (to me it ends upsounding more constricted with less voltage swing & headroom).
You could compensate for that by increasing all of the 220K resistors to 470k to get the load on the phase inverter & the LF roll off back to where you started. But how will the power tubes like having 470K grid leak resistors? I don't know. They may like it just fine or it may cause premature power tube failure when running at high volume levels. Those resistors are usually kept fairly low in value...Anybody have a bead on that one? I would guess that it would probably be fine. However, will it affect the tone? Maybe a comprimise of 330K power tube grid resistors would be the best solution.
So, splitting the signal for a mixed fixed / cathode biased scheme is not too tough. The tough thing is making sure you know what trade-offs you are making and how (and where) to adjust for them if you are trying to keep or acheive a particular tone.
Sounds like fun. I've wanted to do that mod myself but never have. I have an amp that i can switch to fixed on one side & cathode on the other & I like that a lot. Let us know how it comes out. I expect it will be pretty nice.