I think you would find it very difficult to get anything established at the base of the hedge as the soil will be impoverished and full of roots. You may want to consider removing the hedge, grubbing up the roots working in a lot of organic matter and then planting something else.

A few Drypoteris survive under ours. If the holes are in
the lower part you could just plant a shrub in front. We have a Viburnum tinus covering the worst bit. If so expensive t replace a long hedge bu tif its a short one take MG's advice.