Can a Reynolds 753 aero fork steerer tube be unbent?
If by "steerer" you mean the top tube that turns in the bearings, it would be very bad for that to be bent, since it would seriously muck up the bearings, and ever getting it straight enough to NOT muck up the bearings would be unlikely.

Apart from the bearings, with something like a conventional steel fork the twin concerns are fractures in the steel and fractures in the welds and/or castings that hold everything together.  Generally if the bend is ONLY in the length of the metal tubing of one or both fork tines, the likelihood of a fracture is pretty small.  But it's so hard to be sure that there are no beginning stress cracks in a weld or casting that using the fork (with or without straightening) is risky.

If an attempt is made to straighten the fork, it's important that it be done carefully, in a way that will not put further stress on welds and castings.