The script is difficult to typeset, due to it's nature (vertical, left to right).
Why would that make it difficult to typeset? Vertical, right-to-left was once the most common way of printing Chinese. I still come across books laid out that way, in fact. What would be so challenging about reversing the direction of the columns?
I'm talking about for web browsers and stuff like that. Internet websites. I've never seen a Chinese or Japanese website with vertical text.
By the way, the current version of Internet Explorer already supports vertical displaying. It's in fact one of the few things that Internet Explorer can do better than Firefox. I already can assure you it does when I saw a blog about Phags-Pa months ago with the examples properly coded vertically for the font.
I didn't know there was unicode support for 'Phags-pa. Do you remeber the name or URL for that blog?
You could theoretically tip everything 45 degrees to the right. Then it would be right to left.
Mirror it - and voila! It's left to right, just like the Latin script.
Would probably be a bit weird for the Manchus, though.
It would like kinda like Arabic.