Hardsub in BT2020 HLG, what can I do?

[FranceBB]

Hi,
here I am again with yet another issue.
A content provider sent us more movies in MJPEG2000 12bit HDR PQ BT2100 at 23.976fps.
I had to speed up + pitch adjust to 25fps and duplicate to 50p for air time + convert to BT2020 HLG via a LUT approved by both parties (i.e us and the production company).
Now, here's the thing: what they sent us is the textless version. Although the movie is in English and we've been dubbing it in Italian, some characters talk in Russian and have not been translated due to an artistic choice. Instead, subtitles have to be added. Here comes the issue: how the heck do I hardsub in HLG?

I can't just put:

TextSub("subs.ass")

or

AssRender("subs.ass")

and call it a day, 'cause they would be hardsubbed in BT2020 SDR which is no good and would not only screw up the waveform but also make the viewer go blind (imagine watching a movie with dark scenes and subs constantly interpreted as 1000 nits!! You're gonna go blind!)

So... is there a way to hardsub in HLG?
Maybe I can hardsub with a blank clip, apply a LUT and overlay them to the HLG source?

The other way would be to convert the subtitles to .scc or .itt and hardsub them using Colorfront Transkoder, but I'd like to stick with Avisynth.

Any guess?

Crappy workaround: in an HDR PQ 4000 nit source, if I hardsub normally I get the subs over 10'000 nits, so I'm lowering the nits by dimming the brightness of the sub. It will look "grey-ish" in the screenshot, but it's gonna look white in an HDR monitor: Original PQ 4000 nits source - Subs 10'000 nits - Subs 8'000 nits - Subs 6'000 nits

Note1: I also have to fix black levels not just white
Note2: any better idea than this crappy workaround?

[wswartzendruber]

Can you set the SDR brightness used to render the subtitles?

[FranceBB]

Originally Posted by wswartzendruber

Can you set the SDR brightness used to render the subtitles?

I used a crappy workaround: I encoded a low res low bitrate video and marked it as BT709 (wrongly).
Opened it in Aegisub and I used the color picker to make the subtitles with the very same luma and chroma used by the original ones in the original language in BT2020 HLG. Of course I was seeing everything grayed out and dimmed down 'cause it was wrongly decoding it as BT709.
Anyway, once I got them to have the very same levels of the original ones, I did what I had to do and I hardsubbed the original BT2020 HLG masterfile calling AssRender().
As much as I don't like this whole thing, it worked:

Img1: Aegisub - MPV
Img2: Aegisub - MPV
Img3: Aegisub - MPV
Img4: Aegisub - MPV

a few more pictures in MPV after I hardsubbed the file:

Screenshot1 - Screenshot2 - Screenshot3 - Screenshot4 - Screenshot5 - Screenshot6 - Screenshot7 - Screenshot8

Looks like the trick worked and the reference white is good 'cause I copied the levels of the original BT2020 HLG signs/typesetting/subs that were originally in English, so... it worked.
I've sent the file to the QC Department, let's see if they have something to say about them, but if they don't, then I guess it's gonna be aired.

Avisynth Script