You can make a Staff Style for this. Under "Define Staff Style", click "New" and check the Transposition Box. Set the transposition to be the same as your Horn stave and additionally check "Set to Clef" - choose Bass Clef. Apply this style to the score only.

It's a good idea to note this in some fashion—even if you think that no one other that you is going to see this score, preferably on each stave on the first page.

Experienced conductors will transpose automatically. These kind of scores drive us nuts.

Dear Bruce,

Thanks for that advice. Only portions of the score need to be in a different clef from the part, so I'll see if the solution you propose allows this.

Dear Mike,

Thanks for your advice, but I think a note is probably not necessary- even though people are certainly going to see it. Not all the score is to have horn in bass clef, only those places where it is appropriate. As for the question of experienced conductors, this is entirely relative. It seems to me that if it drives a conductor mad to see a score in C, this shows his/her ample experience with transposing scores, and lack of experience with scores in C, which are entirely normal in contemporary music.

Instead of staff style (which works nice): In the part's score manager (sic!), check the box "Unlink All Instruments in This Part". Then you can apply any transposition/clef to this part only.

Thanks Skjalg!  That sounds like a good method.