From the report linked to above:

From the total energy estimate and the electron energy, we can get an estimate of 4 mC for the total charge in the beam. If we then assume that the beam is spreading out linearly and covers a 1.4 cm radius circle at a distance of 4 cm

This is quite a big area – so I would think if the it were replaced by a fine mesh, rather than a simple hole, that let most of the electrons pass through to another small chamber behind the anode, they could be slowed down as you describe (extracting their energy in the process).