I have done quite a bit of that over the years with a 40% CP molasses based product from Westway.  My bales are tight 4x6 and I tip them on their side.  Can get about 4 gallons in them without significant waste.  I let them soak about 1/2 to one hour depending on the nature of the hay.  There is usually snow under them so I can tell when it is starting to leak through when I tip them up to take out to feed.  With more than 4 gallons it starts leaking through while there still are puddles on the top that you loose when you tip them up to feed.  But the cows will get addicted to that smell so even one gallon (maybe diluted with some water even) will get them to clean up less palatable hay.

Last year I bought some tight cornstalk bales and I was able to pour on a day ahead with out any seeping through.

I pay 16 cents a pound delivered in  semi-tankers.