Currently, when encrypting with GPGME, you either hide all key IDs or none of them. This is a problem for MUAs that are trying to safely Bcc people.
fab8b1a166ff indicates at least one possible path forward.
Another path would be a new _encrypt() variant that takes a list of hidden recipients in addition to its list of non-hidden recipients.
This latter approach seems simplest to me, as a MUA developer, and i would be happy to use it.