Coincidentally I did a small test with _HM bitmaps two days ago.
- Added a _HM to the normal inland tiles, instead of just the coastal tiles, mainly by changing the _Data.ini.
- Raised each tile's HeightMapScale from 40 to 240, to exaggerate the effect.
It works as expected, see test image below. A coast like the cliffs of Dover while inland it is like Verdun 1918.
So again, for coastal tiles this stuff is useful. But for inland tiles I have no good answer yet. Maybe some tiles can benefit from a _HM bitmap, like tiles with rocks and snow. But in general they don't seem to add to the viewing pleasure there: Either you don't notice their effect, and when you scale them up it quickly becomes too much.