How do you develop an effective communication program strategy when most teams do not effectively plan ahead (and thus most comms are either ad-hoc/triggered)?

This is such a tough one!! It's so hard when the go-to is to operate adhoc.

In my view, there is power in building a plan yourself. Sometimes the reason comms are ad-hoc is that the person running this program is a creative but pretty awful at organization. In this scenario, if you built a structure that could help the team be more programmatic, would they be open to following it? 

If no, could you start with one peer where the two of you work in a more intentional fashion?

I find that most of us natually gravitate around a person who comes in with a plan. Thus, I think you may have a chance at building a more effective communication strategy if you are up for building the structure and getting the rest of the team to buy into the idea.