My issue with this is that if he's too slow/fragile to pull off Rest+Lum, he's too slow/fragile to pull off sub+sitrus. I say that from experience using the exact same strategy with Exeggutor in NU. If Trevenant doesn't have the defenses to avoid a 2HKO, neither sitrus and a painfully slow substitute, nor Lum+Rest is going to save him. The biggest problem with sub+sitrus is that it's horribly unreliable, with only a 50% chance, you can't just assume you'll be able to sub all day long. Whereas with Rest, you get to full HP which should be enough to make you last another two turns, giving you better odds that your berry will be back. Not to mention, this strategy completely protects you from status.
However, it's a good point that Trevenant is too frail, even with full defenses, to last in OU. Also, it looks like spinblocking is going to be MUCH less important now with the Defog buff. Because of this I don't think Trevenant will be able to cut it in OU without a real role to fill. In the lower tiers, a Lum+Rest strategy will work much more effectively due to the lower power.