Modify a Magnum for LA rocker gear oiling ?

Yes that can work if you drill the passages and use LA heads.

You can also pull the oil from the back of the block by tapping into the oil sender line.

Note this will provide full time oiling versus the LA blocks feed off the cam bearing which is ~ 6 degrees of rotation. You would want to reduce the passage size or use a carb jet to reduce the feed size to the shaft.