Re: A POA question

If you mean putting the tip of the tongue tip actually up into that space, I
don't know of any languages that do that.  It seems to me a little difficult to
do most sounds there -- you almost have to make a full closure.

There are, however, linguolabials, where the tongue is articulated against the
upper lip (on the surface of it, rather than behind it).  As far as I know, that's
the only way natlangs articulate the tongue and lips -- since the lower lip can't
be used with the tongue to block any kind of airway.