Depending on the radio you may be able to use the steep side skirts of the
AM filter as a form of slope detector for FM. I've done it using an Icom
IC-706 ham rig. The problem is you have to be able to tune significantly off
the channel center frequency to put the signal center frequency on the skirt
of the AM filter. Normal unmodified CBs don't allow such frequency sliding.

I've done it with a regular CB. I just turn the clarifier to either edge. It isn't
perfect, but quite understandable. My radio has a narrow receive bandwidth
compared to some radios. I'm still using my TRC-451 (Cobra 146). Other
radios I've done this with include a PC-122 (TRC-465). All stock. The last
thing I want is my transmit frequency to change after spending good money
on a PROPER tune-up to factory specs. No ruined clarifiers here. If I want
to go off-frequency, I have a 2950. It's clarifier is also locked. It shifts in
100Hz increments. Why chop it up for a clarifier mod?