The amount of energy lost in this circumstance is too small to invest effort into reclaiming it.

It is unlikely to be both cheaper than lead and equally reliable.

One day we may find a commercial use for this, but replacing lead with fungi in this context is not likely.

Or you could go with a substance that both stops radiation AND extracts the energy, used in every nuclear reactor: water. It's not quite as good as lead but pretty good, and it will heat up while stopping the radiation, then conveniently through convection can transport itself to a point where the energy can get extracted.

Water extracts thermal radiation at the infrared wavelength, not the higher energy wavelengths that these fungi eat. (X-ray, and Gamma).

Funny enough, these little critters are actually found in reactor pools.