I thought of developing in a tray with the solution, but that's a lot of ferri all in one place, and I don't have a huge amount to play with (though it's not very expensive, and I'll probably get more when I order chemicals Real Soon Now). Worse, I can't find the 5x7 trays I thought I had (I'd bet they're off looking for my 9x12 cm to 3x4 inch plate adapters, which I haven't seen since I moved, despite now having film sheaths for them).
I think that the traditional method for cyanotypes shows an advantage when it comes to development. The two solutions are on the paper. With brushing on the ferricyanide, you open the possibility of uneven development. Perhaps if you had a super saturated solution of potassium ferricyanide that you developed your print in? That might take care of washing away the ferric ammonium citrate to early. Just a suggestion.