One thing hasn't been mentioned, ss1. The dangerous part of the fouling is the salts. They dissolve readily in water. So if you flush the barrel with water (hot or cold, soapy or not) drawing water into the breech through the nipple hole & flash channel using jag and patch with a pumping action, those salts will be flushed out.
What remains is mostly harmless carbon. Yeah, it looks bad on a clean white patch and yes, we usually go after it with more elbow grease; but, for the most part, it does no harm. Quite possibly the extra scrubbing harms the barrel more than a little carbon. Like most here, I run a pipe cleaner up and down the flash channel to get the heavy stuff and give extra attention to the nipple threads, but beyond that I don't worry about it.
The breechplug's a casting anyway with thick wall dimensions and no exposed finely machined surfaces (other than the nipple threads which get cleaned). A modest amount of surface rust really won't have much impact on it.
I've had good luck with bore butter as a rust preventative and go along the bore seasoning regimen. But I question how long the bore stays seasoned when soapy solutions and/or alcohol are used for cleaning. T/C compares the seasoned bore to a seasoned iron frying pan. I know from experience that a seasoned frying pan loses its seasoning when washed in hot soapy water. So is the seasoning only good 'til the next cleanup? Will swabbing the bore with windshield washer/isopropyl alcohol mix prior to shooting remove the seasoning? It seems like seasoning is just another variable that's hard to control.
One man's 'seasoning' might be another man's 'Bore Butter buildup'. I do recall a T/C tech remarking that people tend to go overboard with Bore Butter.
On a couple of barrels, I've shifted away from Bore Butter as a final protectant and am using LPS2 and LPS3. They leave a waxy protective film. I'm sure they'll do a good job protecting the steel, but the jury is still out on whether the hard asphalt-like petroleum fouling will appear. Hopefully swabbing with ws-washer/alcohol before shooting will clear all this away.
The comments about olive oil and petroleum jelly being good for rust prevention are interesting. I'm a bit wary because I got into trouble once experimenting with common cooking oil as a protectant. Got a barrel with a rough section in the bore to show for it. Would petroleum jelly be a 'natural' seasoning?
FWIW
Bob