I got it working
My approach so far involves having a GameObject with three scripts attached to it. The only thing the user needs to do with these scripts is to set the port variable on one of them to the port that you want to listen to OSC messages on.
Then in PlayMaker I have created an action where you set a string to the OSC Address Pattern you want that Action to work with (e.g. /slider1 ), and when a matching OSC message arrives its value will be used to set a FSM float variable. This will happen for as long as that state is active. A FSM event is triggered when a message is received, so you can move to another state as soon as a message with a matching address pattern is received if you want to.
It seems to work quite well. I can have as many of these OSC Actions active as I want, listening to many different OSC messages and updating many variables continuously or as triggers to change state. Limitations at this point, which I probably won't get round to fixing myself at the moment, are:
It will only listen to one port (but this is fairly normal for OSC apps)
A game object with scripts on is also required rather than being a pure PlayMaker solution (a purer way would involve moving more script code to another PlayMaker action that you would use just once to start the OSC port listening stuff)
You have to be careful not to have more than one active OSC Action listening for the same OSC Address Pattern at once (its fine to have more than one action within a single FSM listening for the same OSC Address Pattern, so long as they are not both active at the same time, e.g. if they are attached to different states. But if you were listening for the same Address Pattern in several different FSMs where both actions could be active at the same time, there will probably be problems with my system).
Anyway my initial test OSC Action only uses the first OSC message value, and treats it as a float, so I should probably try to add a bit more functionality to this before sharing it. But I will still try to post it later today