It occurs to me that there is some overlap here with the idea of custom flags. For that matter, I think this could fit very well into the flagging workflow in general.
Perhaps (the option?) to expose or link PM warning to custom flags should be considered as well.
For instance, if “Low quality posting” is something that the moderators want to help discourage, then it’d be helpful to make that visible in the Flag modal as well.
Then, agreeing with a flag, could be part of this new workflow where the moderator would be able to easily “Agree and send warning”.