certtool accepts configuration file using --template option. The format of the file is documented in doc/certtool.cfg in source tar ball. Please install the file into /usr/share/doc the same way as you install TODO and other files. Please note the file is not installed by the gnutls build system.
Will probably do, but with revbump, later on today. Might also send a patch upstream to get this installed...
Added to gnutls-3.1.5-r1, thanks!