When sending an email to multiple recipients who are all on bcc, instead of them receiving it with 'undisclosed recipients' in their 'to' field, just put their email address in the 'to' field.
They should get the email as if it weren't bcc and I can't think of a reason why it shouldn't be done like this.
You can make it an option in settings if you like.
It may be that this is a 'standard' that all email systems do, i.e. put in undisclosed recipients for bcc addresses but protonmail does this also for protonmail email addresses that are bcc'ed.
I'd rather recipients not be aware that I am sending this to multiple users that I don't want them to be aware of.