I probably wouldn't call Silver Bell an OT tune. But it is a tune that I love to play. I learned it from Will Keys and it stays in the same key throughout. He (and I) take a varient of the coarse part up the neck above the fine part but it's still in the same key. C or D for me. I think Will plays it in C.
A chordal approach to a tune while only hittin' the high spots of the melody allows for easier use of standard tunings. The more melody you want to put in there the more you're going to hunt new tunings. I like to get all the melody notes that I can.
When you do hear a bluegrasser change tunings as in Ruebins Train the difference really stands out. The texture is completely different.
I think that most OT musicians are willing to accomidate singers' needs for certain keys. It's just that a new key or tuning will change all his/her fingerings and likely will take some time to work out the tune. So, a wide open, straight ahead OT tune jam may not be the best place to ask him to do this. Catch your favorite fiddler or banjoist away from the crowd and ask 'em to try playing some songs with you.