Since RP'ing is not allowed here, has this booru ever experienced cases of Bloodninja levels of "roleplaying"?

You know, the "robe and wizard hat" types of roleplaying?

In other words, trolling PRers? And have there been users that were flagged for RP'ing AND trolling for this stunt?

Nope, any form of roleplay (except for comedic ones) is frown upon here.
Even those made in response to another person's RP comment.

As I quote the Code of Conduct:

• ...There are acceptable forms of role-play, typically that which reinforces a point, creates humor, or otherwise does not cause a disruption. Even in this setting, however, role-play must be just a single one or two line comment, and cannot be made in response to another role-player.

You will most likely get warned for feeding the trolls/trolling for doing so.