how to make custom combining diacritical marks for arabic letters?

[dinar qurbanov via Unicode]

how to make custom combining diacritical marks for arabic letters?
should only font drivers and programs support it, or should also
unicode support it, for example, have special area for them?

as i know, private use area can be used to make combining diacritical
marks for latin script without problems.

but when i tried, several years ago, to make that for arabic script,
with fontforge, i had to use right to left override mark, and manually
insert beginning, middle, ending forms of arabic letters, and even
then, my custom marks were not located very properly above letters.