I don’t know. I’m focused on @dspreadbury saying alt/opt+up/down can access hi-hat open with the little “o” above them stem the way MIDI keyboard entry can. What he said suggests it’s somehow baked in.
Unfortunately the alt+up down arrow shortcut during note input doesn’t seem to work for Open (2) Hi-Hat. This is because (even if you can define, inside the HH instrument, a new playing technique that triggers the Open (2) sound, and this will indeed work with the alt+up and dow method in note input) you cannot define a notehead for that Playing technique that has the little circe glyph on it (so in this case you have an Open 2 sound but you can define only the notehead, but not the circle glyph above it, as I said) *
EDIT: just realised that @jesele already mentioned this limitation 
* :
So the MIDI keyboard method, or the Playback technique added to the note (during note input or after the fact) seem to be the only ways.
This seems to be confirmed by this video
that was streamed a couple of hours earlier today by @John_at_Steinberg . See at 50:40 (for the MIDI keybord input using the Use Percussion Map preference) and 52:22, where John confirms that inputting open HH “would not work” with the Use Staff Position preference (or, I realised, with the alt+up and down shortcuts, but I may be wrong, @dspreadbury? , and by the way marvellous job answering the chat questions today! among the other billions things you have to do!).
Anyway I suggest the above mentioned video, where is explained another method to assign for example Open 2 to HH, using the Key Editor: just select the notes and drag them to the Open 2 lane:
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Lots of helpful information on this post. Thanks!
Just to clarify, it seem like what @dspreadbury described here applies to other techniques, but not for open hi-hat. Maybe Dorico 6? 
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Yup, that was my point. I’ve been wishing for this to work for many years now.
Not just the hi-hat but any playing technique.
Jesper