The funny thing, as usual, the solution came to me during the night. I went to bed, and I couldn't sleep

All GPGME operations worked ok from withing a shell.
But not from within Apache.
Ergo, Apache was not providing GPGME with some necessary environment element, say a variable.
So, I hacked my Apache's init script, and exported GPGHOME=/var/www/.gnupg, et voila!

Apache's SetEnv didn't work. I should check putting it into the global configuration, instead of the virtualhost container. Or maybe I could set it from withing the module itself, before creating a gpgme context...
So, I believe I will be releasing a new development version of Enigform and MAO this weekend.