Thank you very much for your answers.
Then I think there's still some hope

Approximately 1/3 of the players use an open source client to play the mod (Adrenaline Gamer). I have zero experience in C++ and how a project of that kind is organized, but I think this is the implementation of the MOTD in the client:
OpenAG MOTD
Maybe this is out of the scope of the thread/forum, but doing a fork of that repo to modify the MOTD implementation and adding for example a basic HTML or even something simpler as Markdown renderer would be enough for this to work? Maybe I would also have to include a client cvar that the server checks to see if the client can render the MOTD, and else fall to a default plain text MOTD for clients that don't have that cvar (ie. closed source client).
Also, could I change the 1536 char limit for the MOTD? there's a MAX_MOTD_LENGTH constant in the same repo in the
hud.h and
multiplay_gamerules.cpp,
but I think the reHLDS server
only allows up to 2048? I'm a bit confused about all these different places, would I also have to do a fork of reHLDS to edit that limit so the server can send more than 2048? would all of this be VAC-safe, or a client with these modifications could be VAC-banned?
Sorry if this is out of scope and thank you again!