How could I send control signals between multiple instances of the same vstplugin?

[AdmiralQuality]

Not sure offhand why it wouldn't work in Logic. If the AU works in Reaper, I'd think it should work in Logic too.

And yes, you could potentially put some code at the start of your process function that looks at the current global variable value, and if it's different than its current internal value (saves in a non-global member variable) then calls setParameter on itself to match to the new value. Though still, calling setParameter from inside your process call is probably not the best idea -- but it's your implementation so if you know setParameter is going to execute fast, then sure, you can safely call it. (And note it's not happening once per sample but once per buffer. Which is probably sufficient as long as you're not expecting all the instances to have sample accurate automation with each other.)

Again though, this is messy and a hack.