Quick Answer... don't assign Velocity to Volume. Example:- Element - Amplitude - Level/Pan Page...

Set Level/Vel = 0.

There maybe other places where Velocity is increasing Volume, example in Control Assigns.

Another method might be to use the XA Control with "A.Sw1 On". Make a copy of the Element/Sound, Set the Trill Arpeggio to the Copy and make it XA, so that Trill Element only sounds if Assign Switch 1 is pressed.

Of course the problem with setting amplitude velocity response to zero is that you lose the velocity control. You presumably want the trill to happen at the velocity you're actually playing with.

My thought was applying a square wave LFO to pitch (e.g. enabled by a switch for an instant fixed interval, or a wheel for an interval you could manipulate). Might that do the trick?

One way you could do this, which would limit polyphony, would be to have two elements. One that's trilled and one that is not. Then the mod wheel would be used to volume morph between them. It could be a sudden step change where above a certain point the trill is fully "on" and non-trill is fully "off" - or could be a more gradual cross-fade.

You would use control assignments to control the element volumes. This would assume your trill is a fixed interval.

Using LFO is a good suggestion. There is Element LFO PMD (pitch modulation depth) you could modulate with the mod wheel. Again, either a step change (from zero for half the travel of mod wheel to some non-zero depth constant for the other half of the travel), or a more gradual ramp allowing for controlling the trill more granularly in real time. The step function may serve the purpose better - you have options. This route - using the LFO - would preserve polyphony vs. the "morph" style option given above.

Or you could also have an element that trills that only sounds when you press the [ASSIGN 1] button. Using XA control. And when pressed the non-trill element would not sound. When not pressed, the opposite elements would sound. This preserves polyphony but also requires you to press the [ASSIGN 1] button prior to any notes you want trilled before striking a piano key.

Both Jason and Bad Mister provided a lot of detail (and a downloadable example) in this old thread
https://www.yamahasynth.com/forum/key-doubling-repeating#reply-78771

A lot depends on what method you are using to produce the trill.

Do you have any Effects slots left in the Part, or is the Var Effect of the Performance unused?

If so, there's a lot of good ways to do this with effects.

Velocity is just another Modulation Source.

There's no cardinal rule that it must be applied to Volume/Loudness.

In the absence of Aftertouch on the MODX, I'd say Velocity is the prime candidate for "expressive touch" playing. Ditching Velocity --> Volume is no great sacrifice.

LFO User Curves present another opportunity to set up srumming (or trilling) patterns.

Just to cover bases....

There is always the option of just playing the Trill. Admittedly this requires some technical skill to execute cleanly, but it's not beyond reach.

(Incidentally, as a Guitarist, Trilling to me means rapidly alternating between two Pitches, "hammer-on and pull-off rapidly". I know to others, it means rapidly playing (restriking) the same note... which I know as "Tremolo Picking", such as often heard on Greek Balalaikas. To be honest, I don't know which is the correct musical description... they both come from similar trick bags on a guitar)

Setting the Part to Monophonic allows for very fast Trills if played Legato (but not set as Legato in Edit), i.e. one key is held down, while another single higher or lower key is rapidly struck. On a Keyboard this technique is similar to the "Hammer On, Pull Off" technique on Guitar. The Key that is held down is played each time the other Key is Released (Key Off).... Key On, Key Off.

Mentioning this for the OP..... just in case he didn't already know about this Monophonic option.

Wow, so many good replies and options here. Thanks all! I will try each of them out.
To be clear, what I meant with "trill" is to play the same note repeatedly.

Thanks all! I will try each of them out.

Make sure you check out that thread I posted. In it, Bad Mister provides a download that demonstrates the single note trilling.

Solo element 3 or 4 and see if that is what you are looking for.

You can assign the Mod Wheel to the element LFO Speed and it seems to work pretty well.