Got a Wotsit in his hand.
Άτροπος : ॐ andavane is very insistent in having Macrons over certain vowels.

He is a bit larger than most letters, alas…

In macOS, Option-a followed by a vowel or letter where a macron is possible will produce said combination: ā, ē, ī, ō, ū, etc. You may need to make sure that you have the “ABC - Extended” keyboard selected, rather than the default for English writers.

ā With my computer that is "compose-hyphen a"

This is Linux.

And Windows? I recently got, "I wish there was a compose key on Windows" in one of the chats (I think)...

there are various ALT key codes one can learn for special letters and symbols on Windows.

the accented letters allegedly have CTRL codes but they don't seem to work on plurk on firefox.

with the ALT + 4-decimal-digit number-pad versions ... òó ... and finds those work OK.

ALT Codes for Windows

The alt codes will work on pretty much any OS. But figuring out how to access them may be a bit of a pain. I'd really like an easy way to do Greek, but my options are (as far as I can see) to use ALT codes, or use a Greek keyboard layout.

Thanks. However ॐ andavane hasn't got Windows.
Not used it for well over 5 years now.