havent looked at the patch so maybe it is a dumb answer:

by sending "target 0" to the poly before you change the parameter?

 -110

Welp. I haven't entirely worked it out, but that seems to be working.

Just to make sure I get this: I just send the message "target 0" into that inlet, and then any values i send after that will go to all the voices?

yes, "target 0" means "to all voices".

"into that inlet", not.

into _any inlet of your choice (just use the leftmost all the time), and it will make the poly instances (not just the inlet or what is behind) all listen to input.

the helpfile is a bit cryptic but eventually check the help for "target" message.

example.

"target 1, note on notenumber 60 with velocity 100" "target 2, note on notenumber 63 with velocity 100" "target 1, note off notenumber 60" "target 0, controller data which changes the filter frequency" "target 3, note on notenumber 67 with velocity 100"

now if you use automatic voicing in your poly~, you wont need to use "target 1", but the "target 0" message still works the same. it will not interrupt your automatic voicing, and it has to be sent prior to any new filter parameter.

-110

Instead of sending the message target 0, followed by the intended message for all the instances of poly, I just use pairs of [send] and [receive] objects.

Put the [send] object in the main patch and a corresponding [receive] object in the poly patch... all instances will receive it.