Have you asked the barn owner/manager if your idea of doing all this extra is going to work for them? I'd hate to see you go to a bunch of trouble, buying buckets, setting up some system that still makes more work, so they still want a fee.
Personally, I don't want do deal with baggies of anything.
I bought the pump itself at Costco. Measures out in 1-oz increments. But they can be easily found at restaurant supply, Amazon ... lot of places. I got one for condiments because they handle thicker stuff.
The one-gallon plastic jug is a repurposed Windex jug. The oil I use for her comes in smaller containers. But if you get food oil in one-gallon containers with a screw top, it should work easily.
If it helps, my trainer has a complicated feed system for her horses and most of the boarders as well. Beet pulp, rice bran, hay pellets, a variety of supplements, etc. What she and the workers do is prepare each horse's bucket (most only get one in the AM) the night before, oils and all, and store them in a cabinet. In the morning everything gets soaked. If a horse gets a bucket in the AM and PM, then the PM bucket is prepared after breakfast, oil and all. No problems, nothing going rancid, and since you're soaking it anyway the oil doesn't stay clumped.
I agree that you should have a couple extra hay bags that you stuff and leave ready, as that's probably the biggest PITA, from my experience.
I mix the 1 cup of oil into the alfalfa pellets easily a week in advance (and then bag each dose) and it has never gone rancid etc. Even in summer, but I keep it out of the sun. This way it also soaks into the pellets better so it isn't so slippery oily.
Thank you all for your answers. I have talked to the BO and she agreed it would be much simpler if I prepared everything in advance. So starting tomorrow, I'll get everything ready the night before.
It's really good to know some of you have successfully mixed oil into dry ingredients without it going rancid, the oil part is the messiest and I know for a fact grooms hate it.
The hay part is also a good idea, but because the boarding doesn't include any hay (its charged separately), the grooms are the only ones authorized to handle it.
I've went to the supermarket already and bought 3 large buckets and a bunch of freezing bags (the ones with the zip lock). I'll just put the dry ingredients for the mash plus the oil on the bucket and the grain plus supplements go into the bags, which I'll place on that same bucket. Then I'll pile the 3 buckets. They just have to take the bag out, put it in her wall manger, add water to the mash and serve it. Voila!