Noting that I haven't used Zaino and that the only BF products I've used are their cleaner/polish and sealant (both versions)....
I'm guessing that all the talk about "clearcoat safe" products has you thinking about stuff being/not "single stage safe". It's good to be cautious when approaching something new, but in this case you don't have to worry *too* much.
Generally, it's not like you need to use something "single stage safe" on the cars without clearcoats. Z3 might be optimal for ss, but I'm sure the other Z sealants will work well too (but again, no first-hand experience).
I've used all sorts of stuff on ss (note that I've had numerous cars with the OE ss paint, currently have two) without problems. Generally it's safe to say that whatever works on b/c will work on ss, but the flip-side does apply- you need to exercise caution when using products designed for ss on b/c. After all, the "clearcoat" is just paint without pigment though it is a different technology than the ss paint than they made back in the day.
I've used BF on ss with no problems at all, but some very porous (and/or very dried out) single stage paints might not like it; you might get streaking because it might absorb unevenly. I'd be sure to shake it up really well and to apply it *very* uniformly. I wouldn't expecct it to be the end of the world if it does cause a problem, you'd just have to use a paint cleaner to get it off and try something else. Be sure to do good prep before going to the LSP, the more uniform the surface you apply to the better.
FWIW I generally prefer carnaubas (and even the old glaze with carnauba topper approach) on single stage. If it's really old ss, especially lacquer, I'd go that route. You could always run out the autoparts/autobody supply store for some Meg's #3/#5/#7/#81 and something like #26.