Several ports support a /boot.cfg file. Examples are i386, amd64 (which share a bootloader) and sparc64.
However, they do not share code, nor even the basic command parser. Obviously this is not the NetBSD way to do things and the situation needs to be improved.
The goal of this project is to split the machine dependent parts out and provide generic, machine independent support for most of the /boot.cfg handling, leaving configuration (like what commands are allowed) to the architecture specific code, as well as provide means for overriding command handlers (i.e. implement a common command differently).
Due to the organization of the bootstrapping code this is not as easy as it sounds at first sight, but it does not require deep kernel hacking skills either. Having various hardware available for testing is a bonus, but not required.