According to Will Pattison on YouTube, A-20 is 54% water mixed with ISP and butyl alcohol (sometimes called butanol) that is commonly found in industrial solvents and thinners. I'm not sure if ISP cut with water would work to help with decals, but it might be worth a try: I'd guess most modelers have a collection of spare decals to test techniques with. It's always been something of a curiosity to me that Tamiya recommends A20 for some tasks and their lacquer thinner for others. It was armor gurus Adam Wilder and Mig Jiminez that argued Tamiya is actually a lacquer paint (if you check auto paints and primers you'll often see the term "acrylic lacquer") and that Tamiya lacquer thinner worked better. I certainly agree with Wilder concerning airbrushing results. (Doog uses Gunze Levelling Thinner - a lacquer thinner - when black basing with solvent based paints like Gunze, Tamiya or Mr. Paint.)
Pattison highly recommends an ISP based thinner by a relatively new company in Europe Ultimate Modeling Products. It's available from Hobbyworld-USA or from Amazon UK. (It's $14 but comes in a hefty 270 ML bottle.) If you go on UMP's web page they claim their thinner can be used with any acrylic and have a chart giving the % of thinner to use with any acrylic paint presently on the market. (They claim it will work on Vallejo Model Color - but that paint is bound by polymer which will break apart if thinned with anything. Perhaps a fast drying alcohol would "restitch" the polymer when it dried, but I have no plans to try it when I know thinned polymers like airbrush medium or extender work perfectly well.) UMP is also bottling Badger Stynelrez primer in a multitude of colors.
I have some of the new Mission Model paints and their thinner must be a close relative to Vallejo Thinner which is designed for Model Air because it has no alcohol odor. I tried the stuff last night to re-black my LA7 mule, and it does lay down a fine coat.
Eric