Silver nitrate would definitely be interesting to try. If you have nitric acid based pH down, you could probably put some silver shavings in the nitric acid to make silver nitrate, then dilute that before spraying.
This would probably work to make a nice silver nitrate ion solution (Ag+, NO3-), but it wouldn't have the same properties of a colloidal solution, which has tiny particles instead of individual ions. Silver nitrate has been shown to inhibit ethylene in tomatoes (with long-term treatment, read this-
http://www.plantphysiol.org/content/83/1/44.full.pdf). This means it would probably work to femminize like the colloidal solutions, worth a try right?
The solution would be more reactive definitely, since it has free ions, so if you try this, make sure your solution is really pure so you don't get any wonky toxic salts forming. I don't know the purity of Ph down, but it's an interesing idea.