[Arran]

I don't think that the Philco 20 used electrolytic filter caps, I think they were like the earlier sets like the 511, 76, 86, and my Philco 96 where they used large value paper caps, the same is true of most AC radios from the 1928-30 period. The electrolytic filter caps that came out around that time were primarily wet electrolytics not the so called dry electrolytics, the 1931 Philcos used wet electrolytics originally made by "Mershon" in copper cans. You can replace these paper filter caps by using polyester film types of the same value and voltage, then you won't have to worry about them drying outlike you would with an electrolytic once they are inside the can.
When it comes to the capacitors you are really making more work for yourself trying to troubleshoot which ones are more leaky then others, unless this is just an educational exercise, they are 80 years old and if they haven't failed by now then it's just a matter of time. I would not want to take a gamble on burning out a speaker field coil or a power transformer just because an 80 year old cap might pass the test on an ESR meter, a cap might read fine on the ESR but fail shortly after you start putting voltage across it in service. Capacitors are cheap, maybe between $.25 to $1, transformers can cost $50 on up for a new one.
Regards
Arran