I now did google search with
aim-9+f-35+internal+bay
found in
(dated Apr 14, 2016; part Weapons Integration):
"The F-22 provides for this with an ejection system for the missile, but the F-35 doesnt have this, and this may create problems qualifying the AIM-9X to fire from inside the bay."
and in
"... will be introduced along with more frivolous munitions capabilities (including the 'lock on after launch' AIM-9X as well as the based Joint Strike Missile among other weapons and capability upgrades) as part of the Block 4 upgrade somewhere around 2022, hopefully. "
I think the internal bay is a Block 4 development.
View attachment 28960
The prevailing argument seems to be that the wing mounted AIM-9X will only marginally increases the RCS profile of the F-35A. I saw some discussions in the F16.net forum regarding this but can't remember the details.
Cool that answers the question. Thanks!