How to make fake Black Bar ?

Hi,
I want to add black bars on my Open Matte files. 
Some files in 2.35 or 1.85 exist in true 1.77 open matte (not crop). 
I want to keep the open matte version, but if I want to see it in OAR, can I create fake Black Bar ?
In PGS or other subtitles files maybe ?

[spoRv]

Yes, it is possible, and I did it already few years ago for one of my project.

Copy & paste from that page:

YAO subtitles hack: this kind of subtitle overlay black bars on selected shots; it was tested on MPC, MPV, SMPlayer and should work even on hardware Blu-ray players; to have both black bars and another subtitle, you should use SMPlayer with YAO as first subs track, and the selected language as second subs track

Sadly lost in the HDD crash; IIRC it was PGS based, surely not srt or others of that kind, as you need images of the actual black bars.

Great, do you know how to create this kind of subtitles?
Sorry for your HDD...

[spoRv]

More info here: https://forum.fanres.com/thread-300.html