No. PART "portamento time" is it.

You'll know what can be assigned if you cursor over the parameter and see the [CONTROL ASSIGN] button light up. That's assuming you haven't already filled up all 16 destinations. If you happen to fill up all 16 destination assignments, then you're "full" (no more can be assigned) and the button will not light up even on parameters that can be assigned. This would be not a likely condition - but something to be aware of.

But what you could do is use A.SW2 differently than what you're thinking. Say you have a 1 PART Performance with a synth you want to use A.SW2 to toggle portamento on/off for this synth sound. You could copy PART 1 to PART 2 so you have a duplicate copy in PART 2. Turn Portamento ON for PART 1 and OFF for PART 2. Then use XA control to force the PART 1 elements to sound only if A.SW2 is ON. Then make PART 2's elements only sound if A.SW is OFF (A.SW1 and A.SW2). Then, assuming A.SW1 is always off - you can toggle if you are using the portamento PART (PART1) or non-portamento PART (PART2) based on the state of A.SW2. This "eats up" a PART - but does not consume any more polyphony since only one PART will ever sound at once (minus some possible overlap when switching between and sustaining the previous).

You can extend this idea to other Performances that start off using more than just a single PART.

https://www.yamahasynth.com/montage-category/assign-switches-1-2

On the Montage is there a way to set Assign 2 to toggle the Portamento Master SW On and Off in a performance? Thanks!

You can control Portamento on a per Part basis. If you are going to use Portamento at all, you would set the Portamento Master Switch to On (this works like the Arp Master Switch - it must be On for any Part to use its Arpeggiator)... turning on the Master Switch does not affect anything, until you additionally assign Portamento within one or more of the Parts. It is the Part Switches you can manipulate with [AsSw2].
You can opt to control the individual Part Portamento Switches, as you desire. Leave the Master Portamento Switch set to On. Use [AsSw2] to turn them On or Off. Use the Receive Switch option per Part to opt in or out.

Here’s how:
From your Performance HOME screen press [EDIT]
Tap “Control” > “Control Number”
Set “Assign Sw 2” = 65
Tap the box in the lower right corner “MIDI Settings”
Set the FS ASSIGN = 65
This will work whether or not you actually have a Foot Switch plugged in
We have made your [AsSw2] a virtual Foot Switch
CC 65 is a standard Channel Control Change message for Portamento On/Off

Tap the shortcut box “Control Number” or press [EXIT] to return to your Performance Edit
You will want to check what else is assigned to the Assign Sw2 per Part
Press [PART SELECT 1]
Touch “Mod/Control” > “Control Assign”
Make sure “Auto Select” is active (green)
Press [AsSw2] To recall its assignments...
Decide to keep or “Delete” the other items assigned (if any)
Repeat for each Part in the Performance.
(Assign Switches also often get used to execute XA CONTROL)

You can set the Assign Switch to be either “Momentary” or “Latch” as you prefer.
Press the upper [COMMON] button > touch “General” > Set the behavior of “Assign Switch 2” to “Momentary/Latch”.

On a Part-by-Part basis you can set each to receive the FS or not. The FS is literally turning the Part Portamento Switch ON/OFF, to opt out, set the Part’s Receive Switch for FS = Off
Select the Part, Touch “Mod/Control” > “Receive Switch” > Set ”FS” On or Off as you may require.
Each Part can have a different Portamento behavior!

Thank you! This works well.