With Digitone you could assign an LFO to A/B DEL and get some kind of variation on the note times (I think). Then you can of course save multiple sound variations with different arp settings & times and use sound locks in a track. It’s a shame that arp settings are not stored with sounds on the A4.

The closest thing I can think of for the A4 is modulating the attack of the amp envelope but I’m not sure if that would get you there. Smoothly adjust the rate, I don’t think that’s doable, unfortunately. That would require the rate to be an LFO destination or plockable and used with parameter slides.

Or you can write your own arp in a track, use microtiming, bump the tempo and scale down everything else if needed.