The dust covers all had small holes in them to allow water(and oil, obviously) to escape from the cover. This keeps it from holding enough liquid in it for the flywheel to catch and spit out everywhere. No plug for them, nor is one needed or recommended.
Engine oil leaking from there can be from the rear main, as you suspect, it could be leaking down from the rear of the intake manifold, or even from one of the oil galley plugs or the rear cam plug(rare).
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Joel Adams
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