Ok, here's another take on the topic. Now it's not region-specific and incorporates as many regional diacritics as I was able to find. 
Regards,
Wojtek

2016-11-19 15:20 GMT+01:00 Eli Zaretskii <eliz@gnu.org>:
> From: Wojciech Gac <wojciech.s.gac@gmail.com>
> Date: Sat, 19 Nov 2016 14:21:10 +0100
> Cc: 24967@debbugs.gnu.org
>
> Well, I did consider this for a moment, but the Polish method is called 'polish-slash' and assumes slash as
> the prefix character.

I see.

> But perhaps I could create something called 'polish-prefix' which would serve as a union of scripts,
> i.e. combining ordinary Polish diacritics with regional ones. What do you think?

Sounds like a good idea, thanks.