There was an “isAutomatable()” flag in the past. It was removed from the constructor arguments in one of the latest refactoring of the AutopProcessorParameter classes.
I can only assume, that since there were many hosts ignoring that flag anyway, it made no sense to have it configurable.
The flag is still inherent in the category, e.g. you can’t automate an inputGain (which is communicated to the host via parameters).
It is totally up to you, how you add your values to the ValueTree.
The ValueTree is XML, so you added it using setProperty(), which translates to XML attributes.
If you want it more generic, add your own child nodes to the ValueTree (best not to name them “PARAM”) and set the value as property there.
e.g.
auto node = treeState.state.getOrCreateChild ("Custom", nullptr);
auto param = node.getOrCreateChild ("MyParam", nullptr);
param.setProperty ("id", paramID, nullptr);
param.setProperty ("value", value, nullptr);
Whatever works for you, the host will not read or interpret that data.