I would support creating baroque horn instruments with suitable ranges but not extending the modern horn range to include the range of baroque instruments.

Since you know the ranges of the baroque horns, you can turn colored notes off in the View menu if they really bother you.

its not so much the colored notes that are the problem it is that when doing horn ensemble scores I do them in concert pitch, since there is no reason to do it in transposition.  When playing back these scores, anything from G above the treble staff on up will not play back because it is "out of range".  I have turned off the out-of-range option like you suggested and the notes still will not play.  Thus a horn in C option in the instrument selection would be a great option to have, and with modern descant horns there are scores that do have notes above the accepted range.  On a side note, in the classical period the horns in use were crooked in the keys that a work was written in, and thus there are parts for horn in A,Bb,C,D,Eb,E,F,G, AND Ab.  It would be great to have these options if even in a plug-in. 

To produce the "horn" sounds, you will need to use the SoftSynth or NotePerformer horn rather than the Garritan horn sounds or else find a sound set that provides the crooked horns you desire and which Garritan does not.

There is some sense that the Garritan sounds are now "mature" in the sense that no further wok is being done on them, and AFAIK MakeMusic is not developing further instrument libraries on their own.

You could, of course, inquire about this supposition on the Garritan forum, if you have not already.