An SSPX priest here in Europe told me, that there is a communication from the General House on the subject of requiem.
It states that a private requiem for Bishop Williamson is possible, but not a public one.
"Private" in this context means that the intention for which the Mass is being offered is known to no one but the celebrant (and the the donor of the stipend, or to very few other person). The opposite would be an announced Mass.
Anyone who knows a little about canon law will notice that this order corresponds to the treatment of excommunicated persons. In the Codex Iuris Canonici of 1917, c. 2262 states (in English translation):
Canon 2262
§ 1. One excommunicated is not able to participate in the indulgences, suffrages, and public prayers of the Church.
§ 2. Nevertheless, it is not prohibited:
1.° For the faithful to pray privately for him;
2.° For priests privately and avoiding scandal to apply Mass for him; but, if he is banned, only for his conversion.