How to engrave text or a note below/within the keysignature as Drone Indicator?

[Nils Gey]

Hello World,

I often work with drone-instruments like bagpipes which need to know how to 
tune their drones. In my handwritten notation I included the Drone indicator 
below the keysig as simple text "A" or "A/E" because the absolute notename does 
not matter. 
How do I do that in Lilypond?

Or do you have any other ideas where a "Drone Signature" could be placed? 
Either as Notes or as Text. 

Nils