I have played around some with fillers and loads very similar to what you want.
The most (and really only) successful one was Grex shotgun buffer which is no longer sold.
It was ground up polycarbonate.
One could use BPI buffer instead.
BPI Original Design Buffer (500cc)-Ballistic Products
Or Pufflon which is very messy and has moly in it. It is fine as dust.
PuffLon
My load, which I am still shooting up as I made several hundred, is 4.5 grains PB and 11-12 grains of Grex with a 240 SWC in a 44 special case.
This gives me velocity in the high 600's to low 700's depending on the gun and is very accurate.
This was the result of experiments I did to see how slow I could go and still be accurate without sticking a bullet in a barrel.
When using a filler you want the filler to be somewhat compressed so it holds the powder down and they don't mix.
Filler and wads always increase pressure somewhat.
I have used old tumbling media (the walnut shells will polish your bore!) and even TP.
Don't shoot TP at an indoor range.
It leaves an unholy mess of shredded paper on the floor.
Don't ask how I know this.
Double rifle folks often use a filler in those giant cases that were, in many instances, designed for black or cordite.
Kynoch uses foam wads in all their Nitro-For-Black loads and you can buy those wads if you look around.
Trader Keith
These are very similar to foam earplugs.
What you DON'T want to do is use a solid wad over the powder with air space between it and the bullet.
This is reputed to have caused ringed chambers in several instances including expensive double rifles.
I have a couple 444 cases that have bright rings near where the bottom of the bullet was, possibly from doing this.
Some folks sell plastic disks as wads. Some folks make their own.
These need to be under the bullet not over the powder.
I have given up on wads and fillers for the most part as they are not worth the trouble.
Moving to slightly slower and bulkier powders and staying in the mid 700's to mid 800's velocity gives just as good results without the mess.
I am using CSB-1 and Universal for these kind of loads currently.
It is very easy to move up into the 900's safely with these "mid-range" powders too.
) you can go lighter than 8-grs but don't expect much in terms of accuracy beyond 30-40 yards or so. JMHO. 
