If you don't want to use cement board, then use Schluter "Ditra".
Schluter is a company that specializes in accessories for the ceramic
tiling industry. One of the products they make, called "Ditra", is an
orange plastic mat that effectively replaces cement board, but works in
a completely different way.
Ditra is stuck down to plywood, and then the ceramic floor tiles are set
directly over the Ditra. The shape of the Ditra matting allows the
plywood to move without any stress being put on the ceramic tiling.
'Schluter-DITRA - Schluter-Systems'
Note that Ditra works differently than cement board, but the purpose of
both is to prevent any tension on the ceramic tiling itself. Cement
board is dimensionally stable, and so even if the plywood wants to
expand, the result is that the screws holding the cement board down bend
a bit, and the holes the screws are in stretch a bit, and together they
both allow the wood to move a bit without the cement board expanding or
With Ditra, the bottom of the Ditra mat stretches and shrinks with the
wood, but the top of the Ditra mat doesn't move, thereby preventing any
stresses in the ceramic tiling.
So, both prevent stresses from occuring in the ceramic tiling, but they
do that in different ways.
In your case, no matter what you do, I would nail down a thin material
like 5/16 inch underlayment before you do anything. That way, in
future, if you ever want to remove the tiling you install, you're not
fighing with getting old thin set off of plywood. You can just take a
pry bar and pry up the 5/16 inch underlayment, and any Ditra and tiling
on top of that underlayment will come up with it.
I'm concerned that the tiling in your hallway was installed by someone
who didn't know any more about it than you do, and that you may now be
stuck trying to remove mastic from your plywood.