Cast iron does benefit from seasoning in a yard outdoors. This gives time for the cooling/shrinkage stresses in the iron to release naturally. It gives castings that are very stable dimensionally over time. Widely done years ago for machine tools, sometimes castings were seasoned for a year or two. Probably not done today for most high production volume machine tools.
The process can also be simulated and accelerated greatly by a combination of heat treatment and shot peening with steel shot in a Wheelabrator machine. This process was invented in the 1920's in USA for Model T Fords and is still done today for high production volume quick production things like car engines and agricultural machine parts as well as paint removal and surface microcrack reduction for fatique resistance, on a large scale.
I very much doubt RR did any seasoning of Merlin engine blocks outdoors, due to time pressure – the demand for engines was extreme in wartime. Not sure if they used Wheelabrator machines. Packard in USA, who built Merlins under license from RR, certainly did so on their blocks and heads.
In any case I also doubt if they would bury the blocks if they did have time for seasoning them – removal of soil and risk of dirt getting into passages after seasoning would be good reasons not to bury a block, and the action of soil would not help the seasoning process, in fact it may inhibit the effect. Seasoning outdoors is primarily a heat/cool/repeat process rather than any other effects that happen outdoors. If buried, the heat/cool cycle would be slowed vs being in open air on wood blocks. JD