If I play Heedless Resurrection on my Baron, and resurrect Soul Gorger, can I play back the same Baron with the onplay from Soul Gorger?

Yes — this works. The FAQ #9523 restriction only stops Heedless from resurrecting the very unit it killed, because targets are locked in before costs are paid. Here Heedless targets Soulgorger, so that's fine.

Sequence: kill Baron as Heedless's cost (Baron → trash) → Heedless resolves and plays Soulgorger → Soulgorger's "when you play me" trigger goes on the chain and resolves after Baron is already in the trash, so Baron is a legal choice. You still pay Baron's 3 Power cost (Soulgorger only ignores Energy). Baron Pit is already on the board from the first Baron, so Baron's "as you play me" clause won't re-add or re-route him — he enters at the battlefield you control (or Base) that you choose.