Diki what I posted is in fact verbatim from the FP-50 manual. Here's what it says in the BK-9 manual (verbatim):
“Pianist2”: Same as “Pianist1” while the Hold pedal is not pressed. If you press the Hold pedal, the BK-9 even recognizes “chords” when you press only one note. If the hold pedal is still pressed, chord recognition continues up to a maximum of 5 played keys.
Sounds like exactly the same description to me.

It also says in the FP-50 manual:
"The Leading Bass feature is always on while Split is off."
So it is a LH chord input feature. I agree with you that Bass Inversion should have options for both momentary and toggle pedal operation. Thus both playing styles are accommodated.

Regarding what you are calling Chord Hold, check out this excerpt from the manual of the lowly Roland Prelude:
CHORD OFF: Chord detection in the Lower Part area of the keyboard will be off while you hold down the pedal, allowing you to perform using the entire keyboard. The setting will return to its previous state when you release the pedal.

CHORD TOGGLE: Chord detection in the Lower Part area will turn off when you press the pedal, allowing you to perform using the entire keyboard. The setting will return to its previous state when you press the pedal once again, so that chord detection will be on for the Lower Part area.
Basically the Prelude will do exactly what you're asking. Unfortunately, its momentary chord hold was lost in the subsequent evolution to the BK-series.

The Prelude was an odd duck. While adding this momentary chord hold, it "lost" the desirable Adaptive Chord Voicing which was added in the G70 version 2. [I wonder how the Prelude would transpose a G-70 style track containing an Alteration Mode message!?] It's like the Prelude was designed by a different team based on the pre-G-70 feature set! Thankfully, ACV reappeared in the BK's. But that still leaves Roland way behind the other arranger makers in terms of style control parameters.

If it matters, neither the FP-50 nor the FP-80 has a chord sequencer (chord looper.) However, the upscale model FP-80 does allow the player to jam over any of 180 pre-programmed chord progressions. That's a lot easier than connecting another arranger (or that old Roland CN-20 collecting dust in your closet!) I stand by my original claim that the better Roland FP's are excellent choices for a piano-based arranger.