Too bad its not a little easier to get full "From" headers with mailx. Two possibilities:
Its possible to manually add a customized from message header with mailx on the command line, but for some reason its not possible to set the message header in .mailrc.
I found a workaround here, which creates a wrapper to /usr/lib/sendmail and sets the full name in the from header like this:
#!/bin/sh (echo "From: Full Name <firstname.lastname@example.org>"; cat ) | /usr/lib/sendmail $@