If you're just spraying and not poking and prodding that's likely true, and a lot of less severe hose cracks won't get found that way unless you are pulling and stretching on the hose as you're hitting it.
But the plenum is easy, you just squeeze the body and shake it back and forth. Of course, likely you won't even need the spray because it'll come apart in your hands.
You are definitely right that the E39 is a lot more of a pain to find leaks in than the E30 and earlier vintage cars and more sensitive to smaller leaks. Sometimes the smoke machine IS needed. But sometimes you should be able to look at the plenum and see cracks and just replace the damn thing whether or not it's leaking, because you know it's going to start leaking soon.
--scott