Finally got around to the alcohol test. The storage with the 72% Boveda packets (a la Rosin Saver) works great but after about a year the cake was slightly harder than new.
Stuck the cake in it's silicone cup into a closed jar alongside a wad of cloth saturated in denatured alcohol. Left it for about 20 minutes. Only description is...damn!
While the brick itself was still solid, the surface had completely liquified. Actually was afraid I'd ruined it. But after leaving it uncovered an hour or so, it re-hardened but is now pretty much like new, maybe even slightly more sticky.
So I guess that's how I'll probably keep on...daily storage with hydration packets and annual-ish alcohol adjustments.
Feel a little guity though. Probably won't be buying any more rosin for several years.