Interesting problem!
at is an SUID program. For security reasons, LIBPATH is removed by
exec when an SUID program is invoked - to prevent the caller from
inserting a bogus library! So "at" never sees the value of LIBPATH you
are running with when you invoke it!
I would advise writing the shell script to set the value. You could
put it in another environment variable, and have the shell script be,
for example:
#!/bin/ksh
export LIBPATH=$MYLIBPATH
exec $*
--