IIRC I think we wound up using a slanted half-pipe to get it in and then a funnel for water. The problem with using the funnel for the powder is it gets stuck.

The toilet idea might work. The problem is the foam has to go somewhere and, being foam, it's buoyant and wants to go upwards when water is around. Thus you'll probably need crazy water pressure to keep it from coming back up the drains in the house. If you want to try that, I'd recommend filling 5 - 10 contractor buckets beforehand. After you've put the RootX in the drain cleanout, seal the cleanout. Then pour the buckets into the closest toilet to the cleanout as fast and hard as you can without the toilet bowl overflowing.

Ultimately I think the reason I chose to do things the way I did is the basement in my cleanout area is unfinished, so overflow there is less risky than elsewhere in the house. Also, having drains backup is a sanitation risk.

Anyway try the buckets idea and let me know how it goes.