I haven't actually tested myself, but, the internet ( .edu sites) states that it is ideal by it's self.
This Link
" 1) A carbon-to-nitrogen (C:N) ratio between 15:1 and 40:1. Horse manure itself has a C:N ratio of 30:1"
This Link
"The ideal C/N ratio for composting is generally considered to be around 30:1, or 30 parts carbon for each part nitrogen by weight."
I only have them in a run in shed with a dirt floor and minimal hay wasted due to small hole hay net. I COULD add some sawdust from my cat litter boxes, but that would be urine soaked so not sure what it's C:N ratio would be and what it would do to the general ratio.
Definitely a no on any cat litter added (sawdust or not) the ammonia is WAY to high.
Leaves, yard cuttings and spent garden plants are good to add though.
I have done the pallets as frames myself and found it effective and sturdy.
Any way to fence the horses out of the compost area?