Is there a general radio frequency in Class G for broadcasting intentions? Let’s say you’re in class G and doing some aerobatics. Is there any standard frequency you might use (perhaps MULTICOM on 122.9?) to communicate your intentions?

Assume you’re not using flight following and that you’re VFR.
I'm not aware of any regulation or AIM entry that specifies what frequency to be on.

I use the UNICOM frequency of the nearest airport when I'm out looping because I consider it to be the frequency most likely to be used by traffic near me.

It also has a mild safety benefit -- if something on my plane were to break, I'd already be on the right frequency for the airport I'm about to inhabit.