I thought it might be helpful to have an update, a few weeks further down the line.
What I'm finding, now that my guitars have definitely reached an equilibrium state in their cases as far as humidity is concerned, is that the system (described in detail above) requires hardly any maintenance. Obviously this is primarily because the cases are doing an excellent job of isolating the environment inside them. But I'd expected to have to change the silica gel packs quite often, and although that was necessary at the outset, when the guitars were relatively 'wet', it no longer is. (I should add that one precaution I always take when I take a guitar out of its case to play it, is to close the empty case with the gel packs inside it, to minimise exchange between inside the case and outside.)
Maintenance now consists of checking the hygrometer reading inside the cases every few days - I suspect that once a week is enough, actually. If the relative humidity in a case rises above 50%, I change the gel packs and pop the old ones into a polythene bag. When I've accumulated enough 'wet' gel packs, I heat them in the oven for 90 minutes at 120 degrees centigrade, and that sets me up again for quite a long time.
The upshot is that whereas I expected to have to set up a regular and frequent system of maintenenance, in fact I'm finding there's hardly anything to do to the guitars except play them!
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