Pulled Pork for a large crowd

It depends on what you need the tallow for and whether you want it to taste significantly of beef or not, water rendering will give a cleaner tallow.

With 100lbs of tallow, your main limiting factor is driving off all the water. Tallow is about 75% fat, 22% water and 3% protein so you basically need to boil off 22 pints of water. What heat sources do you have that's going to most efficiently boil off 22 pints of water?

Here's what I would recommend: chop about 1 - 2 lbs of the tallow into 1/2 inch sized pieces to start off with and start them rendering in a pan on the biggest burner you have set as high as possible. Place a thermometer in there and monitor when it reaches 250F, making sure to keep the bottom of the pot scrupulously clean of stuck on bits (if there are stuck on bits, transfer to another pot so that the bottom stays clean).

When it reaches 250F, add in another batch of tallow pieces too cool down the pot and wait until it gets back up to temp again. Repeat until all the tallow is rendered.

This process can be parallelized with multiple pots and thermometers if so desired. The more heat you can apply, the faster this will go.

Brown rice takes considerably longer to cook than white rice, and requires a higher ratio of liquid to rice.

It may be somewhat helpful to pre-soak the brown rice, but that may not completely compensate for the longer cooking time.

The main issue is going to be getting the rice fully hydrated and cooked through without badly overcooking the peppers.

Your best bet is probably to start with cooked brown rice, which my survey having googled "brown rice stuffed pepper recipe" indicates is the most common method, although they don't generally employ a crock pot.

For a one-step fully crock-pot based recipe, white rice may be indicated.