22-5-40
A number of those competitors shooting PP for BPCR long range wipe their bores between shots using a mixture of water soluble oil (available from NAPA as I recall) and water. So in effect, there are a number of shooters oiling their bores.
If you read the forums catering to that type of shooting, such as Shiloh's, you can find what works for powder fouling mitigation when shooting PP bullets.

It also looks like a solution in search of a problem if your issue is hard, baked-on fouling:  if you clean between shots, that won't be an issue...

There's a ton of difference in the way today's matches are shot , and the way the original Creedmoor and midrange were. So yes there were a few shooters that used multiple rods, with the final one with a lightly oiled rag down the bore before the next shot. Some used a fisher brush, which is quite similar to today's bore bigs, or the Bore Wipe System sold by Buffalo Arms.
I like the bore wipe system, followed by a dry flannel patch, it's thorough and fast. I soak the felts in 7-1 water/napa oil mix. 
I also took a page out of the 1875 Remington catalog instructions for loading paper patch cartridges, but as we can't get Spermicetti to oil the exposed patch with, substituted jojoba oil.
Works pretty decent.

Quite a few years ago when I visited the NMLRA range at Friendship to watch the June shoot I spent some time on the slug gun range being shown the ropes by Phil Orem ( now passed )  The loading procedure for the rifles I witnessed consisted of using a false muzzle that had a cross patch pot on top of it and the bullet was pressed through the patch on loading and into the barrel.  I was told at the time that some were using a Teflon backed paper as well.
Perhaps they are doing things differently nowadays I don't know.  Andy Larson should be able to chip in on this subject as I think he shoots slug guns.
Mike.