What ever you decide to coat it with, be sure and convert or remove the rust, because it will continue to oxidize under the coating, even if you're not in contact with the ground. It would be the same as painting over rust, looks good for a short period, until your realize you have defoliation going on under your coating.
In this case, youd have it in the ground and never know its still oxidizing, because you would never see it. If you lucky, you will out live the steel, or in other words, it will take longer to rust out than the rest of your life.
The soil you drop it into will also factor in how long it lives or rots out. Ive seen petro tanks made of steel come out the ground that where buried in a natural sand "vein". They where in the ground for 25 years, bare steel, not coated and they looked danm near new on the bottom half (they where round tanks), because water just ran past them through the sand/soil.
The foundation sealer sounds like a good idea for a top coat.