Eastwood has adapters for their internal frame coating that allow you to snake the sprayer inside. I've not used it but will probably try it out as I get to that stage.
I seriously considered using this to paint the inside. I decided I don't need it and there are enough openings to paint the inside with sponge brushes taped to a stick. This is another example of an expensive method to save a few minutes painting. Just like finding a hot tank to clean off the crud. It would have been more time and money to do it the easy way.