I'm not an expert of Phags-pa, but I found a few printed matter mixing Phags-pa and Hanzi text.
The recommendation assigning U to MVO would be reasonable.https://www.codeblog.org/blog/mpsuzuki/ ... 0523_0.pnghttps://www.codeblog.org/blog/mpsuzuki/ ... 0523_1.pnghttps://www.codeblog.org/blog/mpsuzuki/ ... 0523_2.png
About horizontal writing mode, I don't have good evidence. Because of the vertical stacking ligature of Phags-pa,
the height of the grapheme of Phags-pa would be much varied. To prevent the inconstant linegaps, the rotated text
would be appropriate, in my understanding. However, yet I've not found a sample CJK book including rotated