A solar hot water preheater is in the works for me too. I'd like a wood preheater too.
I had the same setup with a separate 50ga oil fired hot water heater, and that was the only oil-fired thing that ran for a year and a half. We had a 500ga tank, and maybe 50 years ago someone built a stone crypt around it. Then they put rocks and gravel over it and poured a 9' X 5' lid, maybe 6-8" thick. Poured it right around the fill and vent fittings and the outlet tubes. No whistle, no way to add one or inspect the tank. Fast forward to today and the stone wall is crumbing, and I knew eventually the wall would fail and that lid would rip it all apart. So I got a diamond wheel for my circular saw and kerfed the concrete lid into 2' X 2" sections and broke them up with a concrete breakup bar by hand. Did I mention it's on a steep bank with no way to get the front end loader there?
it would have cost way too much to replace the tank and fill it, so I spent $400 on an electric hot water heater. The tank is long empty now and I'll saw it up in the spring (never got to it last summer!).