Surely by now someone has done a simple "port expander" software to be
programmed into another PIC?  Something with two-wire (clock from master
plus bi-directional data) SW addressable units, so you can string a bunch of
them on each pin?  Let's see, a low-model PIC had 12 IOs, which would be two
for the "net", two for "address" (should be able to get 6 addresses out of
that using 3-state connections), and 8 for actual IO.

Send two-byte commands:
        Startbit:UNIT:CMD:data
        CMD :== Set direction | Read | write ...
Get two-byte repsonses:
        Startbit:UNIT:STAT:data

If you only need one direction, it's much easier, of course...

(there are commercial chips (programmed PICS) that do not-too-much-more than
this, but I would think slightly more primitive versions would have shown up
as freeware...)

BillW