Roasting on flames: no
But burying turnips:
1) in hot ashes and embers
Or
2) in the ground first, then burning fire for whatever purposes
Both have been used in yore.
Now with fire mechanics change and us having hot embers: yes please, let us break the monotony of plain turnips in stews and soups.
As for weight reduction, some. But nowhere near the same weight reduction as drying meat.