I think I'd go with Green Frogs thinking. The primer flash would have to penetrate the filler before igniting the powder (I don't agree with the "diesel" type of ignition but IMO I've experienced primers pushing the bullet out of the case before good powder ignition). To be honest I'd have to try several experiments first. Maybe with "blanks", a round with filler then powder then an over powder wad to see how much powder was blown out the barrel...
But in 40+ years of reloading I have never used a filler and only once did I experiment with COW over a powder charge that would "scrub" the barrel on firing...