One possible way to attach it to the 2x12 would be to securely glue and/or screw a block on top of the 2x12 at one end, maybe a 4x4x6 block, and drill a hole clear through the block and the 2x12. That would give you maybe seven or eight inches of wood to keep the rebar upright. A stack of 1x or 2x plank squares in alternating directions would probably be a bit sturdier than a single block with the grain oriented vertically, but I think either one would work out just fine.
riggsp's suggestion (or possibly using a clamp of some sort, like a couple of U-bolts, rather than welding) is also very good.
Make sure you have enough space around the tongue jack for the pole to properly swivel and the flag not get fouled up in jack handles, LP containers, etc.