What is the most effective method for cooking a massive amount of sweet brown sushi rice
I'd cook it in a regular sauce pan, I guess you'll need a big one for your solution. Then add whatever additives you want (rice vinegar, Mirin ect). Then wrap the rice you're not going to use in cling film like onigiri and place in the fridge for the next day. This is a standard method for Japanese people who take lunch to work.

There are big rice cookers but unless you can rent one somehow (Japanese caterers perhaps?) then it'll cost you a couple hundred at least and you have potential problems with voltages and the language difference.

Definitely do not add oil or butter, I tried a similar thing once and it ruined the rice. Fat and Japanese rice do not go together.

The bigger issue are the dishes you are preparing the night before. Unless you wrap it in cling film it'll dry out also for sushi/sashimi you really should make sure it's fresh off the boat the morning it is served and never warms up.