Whilst I can see the use of an artifical device for holding a rifle for chrono or test purposes. I cannot see the point for zeroing. The POI will be totally different to that acchieved in hunting stances.
If the rifle does not free recoil then it can be no other way. That is doubles or bolt or indeed lever guns. Pistols are even worse.
A 375 H+H should not be particually unpleasent to shoot from an upright sitting position at a bench. If it is then the gun fit is way off or it is too light or the loading has been hotrodded.
A quilted sand or shot bag set made "saddle bag" style and hung over the shooting shoulder will take a lot of sting out if a protracted session is contemplated.
As I said to 9.3 x 57 in another thread, I am no fan of recoil, I think that it is detrimental to good shooting but other than my opening remarks I cannot see a use for lead sleds otherwise.

Regards