I looked at a similar thing a while ago. You have probably found this thread https://www.mail-archive.com/therion@speleo.sk/msg07234.html
Yes, thanks for the pointer though, because it means I am not alone. What makes me worry is that I have to modify Therion's own Metapost code for something so fundamental, which brings with it the chance that I might cause some breakage later when Therion gets updated. That particular workaround is pretty cool though, and I would love for it to become a standard feature for all labels, like: point label -style 5 -text "foo" That way, those of us who want to apply a different style to just a handful of labels, could do so without breaking all the others (Stacho's suggestion would have applied to all height labels at once, which is not desired here).
My theory is that most Therion point symbols are defined so that alignments plot incorrectly if they also have a rotation applied.
Quite likely, but for now I stick to the convention in my own symbols, not because it is right, but because it is (more) consistent.