Probably recording 8 different strumming rhythms as "normal" conversion type arpeggios and pick the right arp type depending on if you want triplets etc.   You would have to pick the right arp number.  It won't be automatic although it would be simplified if changing scenes would line up all arps on all parts in terms of the underlying feel.

There's lots of guitar noises that can be thrown in as well.  Pick noises, string noises, etc.   Probably best to start simple and work on adding extras later.  There are arpeggio tutorials that discuss the extras for guitar.

I'm not sure if the docs list the conversation types of arps.  If they did then I would check out what's available as a normal conversion type in preset guitar arps.

Basically, I would like to mimic tremolo picking done on a guitar (fast note repeats performed with a guitar pick on guitar, repeating one note or more notes while playing a chord).

Not a guitar player but I thought that technique is just to emulate tremolo which is an up and down change in volume while maintaining the same pitch.

On a guitar picking must be used so that does repeat the notes themselves INCLUDING the attack phase.

If you just need the tremolo itself you can use a motion sequence to do the job without needing a tremolo effect itself.

1. INIT Normal (FM-X)

2. Turn ON the master motion seq switch and the part motion seq switch

3. Use lane 1 of the motion sequence as the source of a control assignment with part -> volume as the destination

That will give you the tremolo sound. There is a speed setting on the Lane tab of the motion sequence you can set from 0-127.

In that example the notes will NOT repeat but as long as you hold them down you will get tremolo while the notes sustain.

 to trigger simple note repeat so that when I press one note/key (or when playing a chord and pressing two or more keys), the note/notes that I press down repeat themselves simultaneously for as long as I am holding the keys down

Do you really want the notes, including the ATTACK, to repeat? Or are you just looking for the tremolo?

What I outlined was more strumming patterns rather than the classical guitar tremolo technique.  To control some order to the "picking" and also do something different when 2 keys are held rather than say 6 keys would require arpeggio capabilities not offered to user arpeggios (but are available internally).   I would probably start with running through the preset guitar arps and see if anything works.   Any arp should be able to change the timing by using unit multiply (as one lever to pull).  In other words, the preset arpeggios are versatile even when using one arpeggio.