How would one design a (quasi) linear phase adaptive notch filter for a single complex tone?
I might consider doing a complex modulation of the frequency you want to notch down (or up) to 0 Hz.  Then run your favorite deep DC blocker (plus phase adjustment if needed).  Then complex remodulate the result back to the original unshifted spectrum.

DC block: For non real-time, just subtract the entire signal’s average.  Or a long (weighted?) moving average if you can tolerate the latency (Rick Lyons has some notes on this technique).