well i'm still confused as to why you think blasting it is a problem. hundreds of millions of engine blocks have been cast using sand. then the parts are machined which introduces swarf into the oil galleries. before the engine is built up with the other parts (which have also been sand cast) the whole lot is cleaned.

the design of the galleries is such that simple gallery brushes can be used to remove all the rubbish in them. 99% of the sand / dirt / old oil deposits in an engine block come out with 1% of the effort. the last 99% of effort removes the last 1% of rubbish. maybe you dont look for the last 1%?

where exactly does the sand go that you have a problem getting at?