So, I mean... This works :
PARAMETERS SpecifiedRecordID Long;
UPDATE tblRecords R
SET R.LockID = SWITCH(Nz(DLookup("Outstanding","[qryOutstanding]","[RecordID] = " & SpecifiedRecordID),0)>0,-1,TRUE,0)
WHERE R.RecordID = SpecifiedRecordID;
But it seems awfully clunky compared to a "straight"...