In another thread about the lack of subtitles in the Rook Museum, transcripts of the audio were posted.
The transcript for the painting of the "Secret Rook Plans" includes "the only such artifact ever to fall into Glitchian hands — and the only example of Rookish script ever seen".
The inclusion of Arabic/Persian looking writing at the bottom made me think of the Roc from the One Thousand and One Nights, which for some reason hadn't occurred to me before. I think the writing says something like "renounce imagination", which makes sense, but I would be interested to hear from native speakers for a more trustworthy translation. Like a dope I didn't think to take a screen shot but maybe someone did and can post it?
Maybe more artifacts like this will be discovered as the game progresses, shedding more light on the mysterious Rook? Maybe there are other examples out there that I haven't noticed?