I think there is an example diagram in the rulebook that demonstrates this, and allows it
The webpage shows the diagram
www.commandsandcolors.net/napoleonics/th...ics-and-actions.html
EDIT: If the artillery WASN'T on a hill, would the combined arms be allowed? If so, it would make no sense to disallow it because the artillery was on a hill.
So I think the critical criteria has to be that the artillery can ONLY fire in combined arms because it is on the hill.