I eventually managed to prepare the second outer flame-holder in the same way, by flat-sanding the part on some sandpaper, initially quite coarse, then swapping to 240 grit as the part got thinner.
Once the main holes opened up, I only moved the part across the sandpaper by pressing on the center section with one finger - to prevent stress on the delicate outer bits.
I've also managed to remove the excess resin from the inner flame-holders using the technique described above.............. with a little help from a scalpel to cut away the bulk waste first.
Thanks for the tip, Mr Vark.
Well done, ResKit, these are beautiful castings !