PYTHONHOME is defined as 'prefix:exec-prefix'.  Given that definition
the library must be resolved relatively by appending 'lib/pythonX.Y'. 
Given the long standing nature of this interface, I doubt the meaning
can be changed.

It might be possible to _extend_ the definition of PYTHONHOME to have
additional elements, one of which would be libdir.  It seems to me that
this would be backward compatible.  This possibility, and/or other
solutions, would be more appropriately discussed on python-ideas.  And
then of course someone would have to develop a patch to implement
whatever scheme wins consensus (assuming the consensus is that this is a
problem worth solving).