I have added a xylophone to a foot-pumped player before -- it works
fine.  As you pump harder, the xylophone plays louder.  If you use
a salvaged wind-motor governor in line with the supply, the vacuum
remains constant, and the xylophone does not get louder or softer.
I prefer it with expression.

To operate the xylophone in a single-valve player, I tap the vacuum
from the 22 to 24 valve wells (however many notes your xylophone is)
of each of the piano notes that will have a corresponding xylophone
note.  They, then, in turn lift a weighted upside-down pouch which
uncovers a hole, and that hole is tubed to the pouch of the xylophone
valve for each note.  There are problems with doing it any other way,
which I won't get into here.

In the case of a double-valve player, such as a Standard, you can just
run the control tube straight from the outputs of the primary valves,
and eliminate the bleeds in the xylophone valves.  Very easy.

To control the xylophone on/off, just build or buy a pouch cut-out box.
I think Player Piano Company still sells an in-line cut-off box, stock
number 1385, but the hose size is 1-1/4" and you won't need more than
a 3/4" tube to wind a xylophone action.

Buy a couple flanged 3/4" nipples and change them out.  Make sure the
bleed inside is the right size for fast on/off action.  Run the supply
hose into a 3/4" nipple on the reservoir of the player action.  Then
run the 5/32" control tube to some type of sliding lever pallet device
under the keyboard that you can turn on and off at will.  I would be
adverse to a spring-loaded pushbutton as this would get tiresome to
have to continue to hold down.

I have never put a xylophone inside a case of a foot-pumped player with
all of the player still intact.  I have just placed the entire xylophone
unit on top of the piano in full view of everyone to see.  Why bother
putting it inside the case where it never sounds as good with all the
high frequencies cut off?  Personally, I'd much rather watch the
xylophone while it's playing than look at some framed pictures of dead
family members and pets, duck decoys, and beer mugs with slogans like
"If You're So Smart, Why Ain't YOU Rich?"  Wouldn't you?

John D. Rutoskey
Automatic Music Machines
Baltimore, Maryland