Spawn fulltext indexation activity with the same priority as the top-priority activity part of current activity group. And expose the priority of current activity (top-priority for grouped activities). Ideally, the priority of each spawned activity should be the top-priority of all activities member of this activity group for corresponding document. But there is no obvious way to achieve that through indexation API without increasing complexity significantly.
@jm: What do you think about the CMFActivity part of this ? Any other idea ? I feel that this change is so simple that, while non-ideal (cf. commit message) it would be hard (and possibly slower ?) to have a better feature.