Farhan's notion of having a second ladder filter for CW in parallel with the 12mhz SSB filter seems a great solution.
He suggested going down to 1.8mhz or so for the CW filter as I recall,
the lower frequency of operation means the bandwith will be correspondingly smaller.
Should match the 200 ohm impedance of the present filter (or perhaps use additional transformer windings?).
The impedance of these ladder filters is very high for anything out of band, so the two filters do not interfere.
Switch filters by moving the second local oscillator.
I think someone in the forum has implemented this, said it worked fine