What Tootler describes - the use of ground and divisions - underlies some of the Northumbrian pipe variation sets, which are a continuing tradition amongst a minority of players.
They're mostly melodic divisions with occasional forays into rhythmic. The chordal basis is largely defined by the drones in this case.

Some top players improvise them in sessions, in fact there is one who can improvise harmonies to the existing sets on the fly. Some write them down, and learn - composing them for future use.

Paws