I think some of the guard units experimented with them after the Krag was no longer in use by US forces.  I recall seeing a sideplate that was made to mount the Warner Swasey scope on ebay once upon a time.  It didn't look like handyman's work, and I wish I'd bought it!  I also recall seeing a scoped Krag that's in Australia!  The scope wasn't a Cataract, may have been a Stevens, but the mounting appeared the same, or very similar.