I've Heard of Painting between the lines but, can this be done?

I am almost done sanding the entire bottom of my 68 Barracuda from fron to rear (including entire engine bay, inner front fender area, the fenders are off, frame rails, floor pans, you name it) and encountered mostly surface rust. Here is what I have not been able to get at, the inside of the rear valance panel because of the frame, the brace, and the trunk where the spare tire part is welded to the valance. And I can see some rust inside the rear frame rails but the outsides are clean. Will this work:

With a brush/sponge, push and or paint POR15 up into the valance panel (the outside is not rusted)
Put POR15 into an empty and cleaned 409 bottle and spray it into the frame rails through the various holes (the rear sections have rust, not the front) as best I can?

I have done this in my garage because the rust was not that bad, no sheet metal needs replaced, quarters are great, and I had more time than money.

thanks for you advice even if you tell me this is a dumb idea