Add "-b" argument to "cvs rdiff"?

I'd like to be able to do date-based comparisons with "cvs rdiff" on a
branch instead of on the trunk.  As far as I can tell there's no way
to do that.  The obvious solution seems to be to add a "-b" option to
rdiff, which would be ignored for "-r" arguments but used for "-D"
arguments.  My questions:

1) Am I correct that what I'm trying to do isn't supported right now?

2) Is the solution I'm proposing reasonable?

3) Does anybody know of any gotchas I should watch out for when I try
   to do this and/or suggestions for how to implement the change?

Thanks,

  jik