The makefs tool is a very versatile userland application used for various tasks during the build of a NetBSD release. It can also be used to create CD or DVD images, not only for building NetBSD install media, but also for backup purposes.
Unfortunately it does not currently support creating blue ray images in UDF format.
NetBSD does support UDF, even writable, but using mount_udf for image creation requires running on NetBSD (while makefs is very portable and part of the cross-building environment), and also requires pre-allocation of space to the created image.
Adding support for UDF to makefs is easy. The existing NetBSD header files can be used and the file system format is well documented.
Note that you do not need a BlueRay burner for this project, you can create (small) test images from arbitrary data and verify them with tools readily available in pkgsrc.