Recording over video on a DVC

NO

There is a technique to recover data that has been written over on a hard disc, it involves precisely remagnetizing bits and then looking at the residual magnetization.

I suppose in theory the same technique could be applied to a DVR head but that kind of equipment, if it exists, is extremely expensive.

No. When you delete data on a hard drive, you are really just removing a labelling identifying them as protected, allowing them to be written over if necessary. When you write over recorded video on a tape, it's really overwritten.