Interestingly, since we started having midfielders in the 60's (we had half backs and fowards before then), before Williamson ,only 3 recognised midfielders have managed double figure league goals in a season - Currie (10) in 71-72, Trusson (11) in 81-82 and Brown (16) in 02-03. An argument might also be made for Woodward in 76-77 and 77-78 (10 and 12 respectively) as he may have been playing more as a midfielder by then.
In any event 5 of Brown's 16 in 02-03 were pens as was 1 of Woodward's in 77-78, so the record for midfielder league goals from open play in a season stands at 11. It will be interesting to see if Williamson can beat that.
The record for a defender, incidentally, is easily Kenworthy's 15 in 81-82. 9 of his were pens, which I think makes him joint top open play scoring for a defender with Colquhoun who managed 6 as well in 69-70. Lowton with 5 is also closing in on that record.