Some assistance over at eOS finally turned up some useful data. There’s an open bug report over at freedesktop’s gitlab (10654, “Unable to set uncommon screen resolution with Arc 380”) going over the same issue(s) with some sort of resolution in the works:
This is another case caused by the lack of the PLL algorithm.
There is a patch in works over there (135397, “Add HDMI PLL Algorithm for SNPS/C10PHY”) so far that seems to have resolved the problem for some. I’m going to try to implement it when I have the time to.