[NTG-context] place single entry of a register, or filtering the terms of a register

Hello list, is there a way to place only one single entry of a register, or a selection of entries? I looked into strc-reg.mkiv and strc-reg.lua and i would answer "no, it's not possible". I could hack strc-reg.lua and define a command to do that, but maybe a better solution is already there. Here's a very simple MWE to work on, in case it's possible: \starttext Cat\index{cat}, mouse\index{mouse}. \page Cat\index{cat}, dog\index{dog}. \page \placeindex \page Here I'd like to place only the "cat" entry:\par \placeindex[criterium=???] \stoptext Massi