I have a couple of different brands of hose-end fertilizer applicators. I've never tried to put anything in them except the product they were made for, because there is no way to accurately determine the rate of application. Not everything dissolves at the same rate. Unfortunately, none of the hose-end applicators has markings to help determine a general application rate.
Perhaps it can be done, though, and I just don't understand it correctly. I've overapplied fertilizer to my lawn twice, and made it turn brown. I don't want all my shrubs, flowers and veggie plants to have the same fate. I know the grass will green up as it grows out, but I think annual plants might just die from a really heavy overapplication of fertilizer. IMO, it's better to put the fertilizer on the soil, or dissolved in water in a bucket/watering can, at the rate recommended by the brand.
It will have to be someone more adventurous than I to run the "test".
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams