Yes, I have seen that one but it only refers to Chymotrypsin-like activities which is not really an answer (but some sort of it since both proteasomes have beta 5 subunits). I was thinking about simple experiment published somewhere where purified proteasome was incubated with MG-132.....
In De Bruin & Overkleeft, Angew Chem 2016, they do a bunch of experiments with selective probes for the proteasome subunits, and show inhibition with a bunch of inhibitors (Figure 4), but MG132 is lacking. In Florea & Overkleeft, Chem Biol 2010, Figure 4, you may find what you are looking for (competition of beta5i & beta1, which don't resolve on gel, by MG132).