GAU-12 on GR7 / 9
Your guess is as good as mine, obviously it would need the mechanical & electrical attachments, but seeing as the GR5 ( from which all the 7's & 9's are adapted ) was meant to carry gunpods, can't see it would take much effort.
I wouldn't mind betting it's been seriously considered, then whoever was doing the considering got slapped on the wrist for political reasons, mainly company rather than nationally.
Royal Ordnance, who came up with the thing, had just been absorbed by BAe at the time.
We even had mahogany gunpods full of cameras on the GR5 trials !
For that matter,there must be a lot of 30mm Aden packs around, why not use them ? Can't believe they couldn't be made to fit. Not as trendy in terms of rate of fire - the Aden itself was a development of a WW2 German gun - but it's still something not to be on the wrong end of.
I know we're not supposed to mention other websites, but if you were to put 'Harrier' into a search engine, find a large UK site ( brilliantly done by Michael Pryce ) then go to the history section, then scroll to 'Harrier Testing' you might find my masterpiece with relevant photo's...