I am new to this site too, but I am a designer/ engineer. The old pnuematic systems were bulky mostly due to the limitations of the materails and manufacturing processes. Modern materials and hand crafting can result in smaller but effective systems. If you are crafty enough, you could miniturize all the parts and still get the air volume you need for the hammers.
go with an electronic system that is connected to a "bar' pick up for piano rolls. This you will have to make. Instead of a vacuum based bar, the bar is an electical pick-up with a conecctor bar on the outside of the roll. The standard piano rolls would have to feed between the two bars but when the perforation comes up an electrical conenction is made between the bars. One side might have roller conectors that are lightly sprung. From this, an electrical signal is then amplified via micro relays for driving the hammers.
i do not know if this already exists, but if you are doing something not yet done, then why not do something not yet done!
just thinking out loud.