I’m looking for the regs for cables in chimney chases but can’t find it. Any help locating it (in either the IRC or NEC) would be appreciated. Thanks in advance - 
You won’t find it.
Generally, a “chase” is not for exclusive use. It can be shared by several systems/components. However, many times you will find other issues with a shared chase, such as clearance problems, missing or improperly installed fire-stops, etc.
Thanks Jeff- I was looking for quite a while with no success - Now I know why LOL!
Chimney chases are a popular place to fish wires between floors, or from the basement to the attic. As Jeff rightly points out, it can be difficult to maintain the clearances often necessary between the manufactured chimney components inside the chase and the fished wiring. If the wiring was installed as new as the chase was being constructed, chances are pretty good that the clearances have been met. Draft stoppage where the cables enter and emerge from the chase is a point of concern.
Abandoned brick chimneys, such as you will find in great supply in older homes in the northeast, are fantastic places to run new wiring from the basement to the attic. A super-highway for pipes and wires. A person would just need to be sure that the top of the chimney was sealed off from the elements, or that the chimney had been previously torn down below the level of the roof.