I ended up going the unix signal route - once a process starts, it saves its PID to the database.
The process then goes into a while (<>) loop to wait for input (although i might want to change that to something non-blocking later on)
When one process wants to 'speak' to another, it stores the message in the database (could use shared memory or a file for that), looks up the other process' PID, and sends it a SIGUSR1. The other process has a signal handler installed which, when triggered, looks up the data to be received in the database, and does it's thing.
Only thing i'm worried about is race conditions between handling the signal, and waiting for and receiving input, but i think i can reduce the risk of that.