This has been revived because a Collect implementation is now almost ready. 
Thank you, @martijnr, for merging in the ODK XForms spec at https://github.com/opendatakit/xforms-spec/pull/198.
There is a proposal for bringing this to XLSForm here.
I reviewed the @TAB call notes and saw that there was talk of changing the action name to setgeopoint for consistency with the geopoint type. That would be fine by me. I do like that it might leave space for introducing an alternate geo type with a different format as has previously been discussed.
I didn't listen to the call -- was it a done decision or more of an idea for another round of discussion?