What you are asking is possible, but there are two considerations.
The Ethernet cable from the ESS to the computer has better noise
immunity than the parallel port cables. This probably is not significant
in you case, unless one of the machines is a plasma cutter.
In order to get the full complement of inputs and outputs on the
PMDX-126 working, you need to provide 2 parallel port connections,
thus requiring 2 parallel port cables back to the ESS.
Oops, upon re-reading your message I see that you asked if you
could put the ESS and the PMDX-126 in the same enclosure and
"with a PP cable running to the machine". The signals between
the PMDX-126 and the machine are not connected with a PP cable,
but rather to individual screw terminals on the PMDX-126. It is
possible to run these signals through a suitable cable harness
and connectors to allow the box to be moved between machines.
There are no significant electrical issues, but building the cable
harnesses will be a bit of work.