You can't powder coat filler
Actually you can, if it can withstand the 300 degree bake out,( high temp "Lab Metal" filler).......most powder shops are still using 20+ year old equipment, so they're limited to what they can do.....but there are now new "wetting" agents that are sprayed on prior to powdercoating that can conduct the positive charge for the powder to "flock" to, so anything in the past that couldn't conduct contenuity, can now be powdercoated, but the item coated, must still be able to survive a 300+ degree bake/flow out.......and with custom masking agents and tapes, custom patterns are just and imagination away as to what can be done, some of the more updated shops can even powdercoat automotive glass, the techniques and powder medias have become as common as any specialized paint finish out there,......there's actually hundreds of colors, textures, metalics, marblizers, wrinkles, etc, etc, etc and assorted techniques available, just a matter of finding the shop capable of performing what you need
