Still works for me with GREEK CAPITAL LETTER OMEGA U+03A9, followed by the combining dot.
For clarity, the capital omega looks like the one used for a logo by the watch company, and the small omega looks a bit like a curly W.
Aha, problem: I was trying it out in TextEdit, but in InDesign, the dot crashes into the character, isn’t high enough to avoid the top of the capital omega… hmm.
Workaround:
use Character Viewer to insert the capital omega,
click baseline shift (about 3 pt depending on font size),
go back to Character Viewer to insert the combining dot above,
pick a font that displays it OK.
Maybe.
Chris.
Edit after seeing your edited reply: almost as terrible as the solution for the Welsh W+circumflex, when a client insisted that we had to use a font that didn’t contain that letter. We had to insert a floating circumflex and use insane amounts of kerning to float it into position (different amounts for different font sizes), then copy-paste every instance!