Let's face it: the main bad actors in black powder fouling are water soluble, not oil soluble. They respond very well to water, hot and soapy, or otherwise for cleaning. Hot soapy water helps remove the residue left over from the BP lubricant, too.
I have a shop that is air-conditioned and also has a dehumidifier; the relative humidity runs right at 30 percent, according to the humidistat. After about 3 days, I will see a faint reddish discoloration on an unprotected, black powder fouled, surface. (I tried it to see)
Over the years, I have tried all sorts of stuff and plain hot, soapy water is quite good. However, I have lately had really good luck with Ballistol Milk. Whatever is is Ballistol, perhaps a light "soluble" oil, using the stuff as an emulsion allows me to delay cleaning for a couple of days with no apparent ill effect to the gun. Although Ballistol is said to "neutralize" black powder fouling, I see no reason that it should (or does) beyond the simple expedient of an oil cover. The water in the stuff helps dissolve the fouling.
One makes Ballistol Milk by adding 1 or 2 ounces of Ballistol to about 10 ounces of distilled water and shaking it up. Presto, you have Ballistol Milk! I mix mine in a cheap spray bottle of a convenient size.
After shooting, simply place the gun on an impervious surface and spray it, all over, outside and in, with the stuff and walk off if you want. Come back at your convenience and swab the thing down until clean and re-spray. Go off and leave it for another convenient time and wipe it down again. Then store your slightly oily gun as you would normally.
The only real drawback to this is that Ballistol has an odor reminiscent of a sour baby, maybe sour baby poo! It is not bad and goes away in short order, so it is really not a problem