how to make custom combining diacritical marks for arabic letters?

[Richard Wordingham via Unicode]

On Thu, 17 May 2018 08:43:35 -0800
James Kass via Unicode <unicode at unicode.org> wrote:

> This page describes the essentials of OpenType Arabic font
> development:
> 
> https://docs.microsoft.com/en-us/typography/script-development/arabic

But isn't the problem that PUA diacritics won't reach most Arabic
shapers?  I think we're back to the vexed issue of defining Unicode
properties for PUA characters to applications.

Richard.