To update a previous post, if you are using ISC DHCPd, you don’t need to copy startrom.com and others around to alter the path the RIS client tries to use, you just need double slashes:
filename "\OSChooser\i386\startrom.com";
And, I’d recommend Argon’s RISme, though looking at the capabilities of RIS Menu Editor, I’d say either one are perfectly capable of doing the job.