I would agree the techniques was commonly used, very old in tactical thinking, and still used in the 1960-s in SEA area of Ops.
Heavy water cooled MGs were the normal culprit for this, albeit John Browning's excellent design of 1/2 inch bore is an air cooled exception that works well.
See J.E Kaufmann's Book "Fortifying Europe" on how the many , many emplacements incorporated not only the principle, but the ingenuity of it throughout all of Europe in the early 20th Century. Most impressive for me was the indirectly aimed via a cartographic device method through a fully enclosed embrasure ( the defense against FT assaults).
All sides used indirect grazing fire for the area denial role from 1917 on.
The primary purpose of such fire is in fact area denial. As such, one should focus the rules accordingly for the effect. It is not aimed directed fire at a specific enemy location or geographic point. ( IF it is this which is being desired, why not just follow Rob W's idea of placing the MG in play on an off board overlay?)
Ma Duece was still used in Vietnam on firebases for the same purpose, shredding foliage in order to deny an easy route of advance into concealment terrain by the enemy forces. It was likewise used in Fallujah to sweep rooftops during the assaults in order to deny the height advantage to enemy snipers from long range non directed fire.
The principle is sound. Ballistically, the gun is pre measured for bullet drop point of impact at ranges over 1 mile, then elevation markings are set on the gun frame for this graizing fire to be fired upon command; and a set of aiming stakes goes to each side to delineate to the shooter the boundaries of such fire left and right. since air friction will slow the bullet down eventually to a point of no force upon impact sufficient to inflict harm; there does need to be some focus as to the ultimate effective range of the grazing fire.
KRL ,Jon H