While using the Burning Crusade Classic version of the addon there is no option for Shadow Priests to change which bar appears when they enter Shadowform. In TBCC Shadowform is a stance and, when using the default UI, will page action bars accordingly.

I believe that GetNumShapeshiftForms() is returning zero for priests. I'm not sure where it is defined though. Still digging.

Yep, GetNumShapeshiftForms() is returning zero for priests. Just found the API doc for it and tested with a print statement. Seems I was mistaken about the client fully treating this as a stance despite paging the action bar like one.

I tried adding the following to getStanceTable() in options/bar.lua but nothing changed in the option menu:
if class == "PRIEST" then
tbl[1] = GetSpellInfo(15473); --shadowform
end

idk where to go from here.

I think line 343 is keeping it hidden even if the stance table function gets updated.
hidden = function() return GetNumShapeshiftForms() == 0 end,

I tried to do a quick hacky edit but it didn't work:
hidden = function() return ((GetNumShapeshiftForms() == 0) or (class == "PRIEST")) end
I tried to reuse the 'class' variable declared outside and above the stance table function on line 156 but I probably dont know enough about LUA to do this right.

My next idea would be to see if there is more than one item in the 'values'

digging thru patchnotes revealed that, although blizz said Shadowform gives new bar 'like stealth', it is not being treated the same with regard to api calls. 2.1.0 was the patch for that. 3.0.2 added the shapeshift elements.

I dont know enough about the wow api to test what event is being triggered when going in to shadowform that causes the bar swap. Patch notes make it sound like something that was hacked in and changed later to something that follows established patterns.