Could you allow this to be a long-term search only for registered bump lists?
A better solution might be to make the lists something special, give them a profile page and automatically bookmark all threads that people flag (and not allow for double-bookmarking). You could keep all bookmarks for X number of days (maybe 30 or so), if people spam the lists, you could treat it like any other kind of disruption.
The lists could also be given a separate, permanent set of bookmarks for very important articles the listkeepers want to keep for a long time (this bookmark list would function exactly like the regular bookmarks on peoples' profiles).