I have found only one use for the old Zinsco panels. The cans (with the addition of a few screws) make good Jboxes. Forget the Chinese breaker ... been there, done that. It lasted about 8 months, tripped and wouldn't reset. The "Boneyard" in Alexandria, VA sells "reconditioned" Zinscos (if you want the telco, drop me an email). I believe their definition of "reconditioned" is, as gfretwell says, washed and dried. My vote would be to get one large panel and sell the Zinsco guts on eBay.
I have had good luck with a company from California, www.relectric.com. They have Zinscos, reconditioned with a 1 year warranty. I bought some discontinued/reconditioned Westinghouse breakers from them, been in for several years now with no trouble. They claim that the recon. breakers are tested, but I have no idea about the testing parameters.
I've seen those claims as well: "tested to industry standards." The claim is pure BS. Part of the problem us that there is NO way to effectively test a breaker.
Oh, there was once an ANSI standard that had something to do with testing breakers. The problem was, the standard only addressed certain aspects, and was quite specific in stating that it was NOT intended to be used ro check used or reconditioned breakers. Over at UL, it is necessary to destroy a number of breakers in the testing. After a breaker is 'listed,' quality control tests at the factory are complimented by on-going destructive tests.
Whether ETL is as relaible as UL is another story; considering the shennanigans that Chinese manufacturers seem to delight in, I suspect that there's a reason they are avoiding the UL follow-up testing.
It occurs to me that a currently available breaker costs only $8, while the Zinsco-type replacement costs ten times as much. Do, I think I will combine approaches: get a 'bargain' used Zinsco off the internet, and back-feed a breaker at the panel. That way, even if the obsolete breaker fails, I still have the panel protected.
I may have a couple Zinsco 2 Pole devices in Storage, which I could let go for a good price. They are from past Improvement Projects, where the Panelboards and / or Gear was removed, and replaced with more current equipment... likely due to layout revisions, etc.
The largest Panelboard Device I would have (a Device which would fit on a standard Zinsco < 225 Amp 240V Bus Kit), is 100 Amp (I think...). Might be 70 Amp, but it seems like there was a 100 Amp 2 Pole Device in my Archives.
If the use of a non-tested Device may cause Nocturnal Horrors, the best option would be to perform a "Sub-Feed Tap", as you have mentioned. That may be the most cost-effective choice overall.
I also agree with the option of ditching the Zinsco Panelboard, and installing something modern.
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!