I agree with LB... and I think this may be a good place for you to get your feet wet in creating some of these plugins if that is what you desire. There are plenty of options already available that you can use as examples. Use some of the existing multi-boot options that already exist as well as the discussion you can find here
http://www.ubcd4win.com/forum/index.php?showtopic=5158 in which Gparted and Spinrite plugins were discussed.
You may want to note that for the hard drive diags... they will often check to see if a drive of that brand exists or not before they will actually work so if you are going to use one of these as a starting place.. make sure you have the brand drive that you are trying to make a diag plugin for.