OMG, that is brilliant. I was spending a ton of time trying to get deeper and deeper attributes from the arp and setValue and diving deep into sliderpack data, but low and behold, the property editor saves the day again.

Thank you @d-healey , brilliant as always!

Does that connection remain once exporting the instrument?

Also, it doesn't show anywhere in the JSON editor for the component, is that correct?

And since we are here, I also have a Midi Processor Arpeggiator, and I created a SliderPack in my interface. For that there is only a processorID but no parameterID. As such it controls the SemiToneSliderPack, whereas I want to control the VelocitySliderPack. Is there any way to modify this?

You point your processorID at your Arp - and use the SliderPackIndex to say which of the arps slider packs you want:

Velocity is 1, Note Length is 2

Does that connection remain once exporting the instrument?

No. The connections are saved with presets though so once the user loads a preset the assigned controllers will be restored. We need to keep nagging @Christoph-Hart to have "true" default presets :)

Also, it doesn't show anywhere in the JSON editor for the component, is that correct?

Correct. The JSON editor shows the fixed properties of a control. A CC assignment isn't fixed so won't be there.

And since we are here, I also have a Midi Processor Arpeggiator, and I created a SliderPack in my interface. For that there is only a processorID but no parameterID. As such it controls the SemiToneSliderPack, whereas I want to control the VelocitySliderPack. Is there any way to modify this?

I haven't used the arpeggiator so I don't know anything about it. However since sliderpacks are a collection of multiple controls it's not straightforward to assign them like knobs, sliders, buttons etc. The usual approach is to use an intermediate sliderpack data object. I have a video about this on my Patreon page. I'm not sure it will work with the arpeggiator though because you need to be able to edit the script to implement it.

Does that connection remain once exporting the instrument?

Also, it doesn't show anywhere in the JSON editor for the component, is that correct?

And since we are here, I also have a Midi Processor Arpeggiator, and I created a SliderPack in my interface. For that there is only a processorID but no parameterID. As such it controls the SemiToneSliderPack, whereas I want to control the VelocitySliderPack. Is there any way to modify this?