Well I asked my brother in Edinburgh to check his copy of Boswell's Edinburgh Journals 1767-1786; several days later he has reported back that -

"got distracted by all sorts of interesting digressions in Boswell's Journals but no, nothing about him fiddling or playing any other instrument. Any spare time not spent drinking, womanising, theatre-going, conversing and corresponding, etc seems to dabbled a bit with writing poetry but not music. Though obviously he went to some pretty boisterous establishments where no doubt there was music."

Matthew