Doing the same research as you, I found your topic, only to be bummed seeing it had no answer.
I asked for some help on the Plutonium Discord, and Hadi77KSA was kind enough to share their script randomizing characters in multiplayer! So long as the host has it, no one else needs it. You just gotta place it in this folder: %localappdata%\Plutonium\storage\t5\scripts\sp\.
Posting my research here for future reference 🫡

Works wonder; thank you so much!