Is it possible to assign AF1 or AF2 buttons to a Performance where these specifically assigned functions are unique to that Performance (not the AF assignment associated to the Patch within the Performance brought over from Patch mode)?

The [AF1]/[AF2] buttons are available in every Performance; what they do within that Performance is always unique to that Performance because the assignment is made at the VOICE level of programming (not Patch, “Patch” means something entirely different in the MOXF).

Each VOICE has can have unique programming as to what the Assignable Function buttons do (thus the name). What they share when a VOICE is placed in a PART (like when you are combining Voices into Performances) — the behavior can be either Momentary or Latch — so you must work out the behavior most suitable for your musical goal.

This is true always when you combine VOICES into Performance Parts. The Controller assignments are typically made in VOICE Mode, you can then set the “Receive Switch” if you wish to activate or deactivate the reception of that controller, per Part. For example, All VOICES (except drums) respond to Sustain pedal in Voice Mode... but when you combine a Bass and a Piano in a Performance, you can deactivate the Sustain Pedal (Receive Switch) for the Bass Part. It just makes sense to do so...

The same thing applies to the [AF1]/[AF2] buttons. In addition to the role as an assignable “Source” in a Voice “Control Set”, the AF1 and AF2 buttons can function in Yamaha’s XA CONTROL. (Expanded Articulation)... this means you can use them to make certain sounds available.

XA CONTROL is a way in which the Yamaha AWM2 (sampled based) engine can seamlessly switch sounding Elements while you are performing. You can use these buttons to seamlessly switch which of the 8 potential Element Waveforms is sounding during your performing on the keyboard.

You can use it to switch between a bowed string and a pizzicato gesture, you can use it switch to a guitar harmonic, you can create pitch scoops, hammer-ons, access legato samples (sans attack), or to a pitch fall-off on a brass sound.... you can animate the instrument sound you are playing. It allows you, among other things, to turn On and Off Elements....

This is sort of like an intelligent Mute. A Mute disconnects something from the audio output, this has that effect but it does the job without cutting off the sound... XA CONTROL is sonically invisible, and allows seamless transitioning between the currently active Elements.

So you can continue to hold the Strings, hit the AF button, the strings will remain sounding as long as you hold, but all subsequent notes can be an entirely different sound. Which is exactly what you are describing.

You could set the Performance so that the Strings only played when a particular condition exists

XA CONTROL is set in Voice Mode.
Press [EDIT]
Touch the Track [1] button to view Element 1
Each Element can be set individually
Press [F1] OSC
Press [SF1] WAVE
Set to “All AF Off” = this means the Element will be available anytime both AF buttons are Off. Activating either button will deactivate it.
Set to “AF1 On” = this means the Element will be available only when AF1 = On
Set to “AF2 On” = this means the Element will be available only when AF2 = On

You can prepare entire Voices to respond by setting each of its Elements to respond a certain way.... so that they all sound (or not) based on the XA CONTROL setting.

Create the USER VOICEs in Voice mode, then use them to prepare your Performance.