Tower Controller described event when a B747 departure had crossed the runway hold line; experienced some problem and declined takeoff; CRJ traffic on base was advised of the situation and given a landing clearance.

Narrative

B747 was cleared for takeoff Runway XX. CRJ 7 miles southwest on left base cleared to land. B747 started to taxi toward runway and stopped crew indicated warning lights I the cockpit that needed to be resolved. Flight crew advised they were 20 FT beyond the hold line. It appeared the nosewheel was just shy of hold line. The hold lines are approximately 125 FT from edge of a 200 FT wide runway. Landing aircraft (CRJ) was advised that B747 reported slightly beyond hold line. CRJ crew acknowledged and landed without incident. Initially; Controller handled same as mowers working adjacent runway within the safety area. That is allowed as long as mowers are on Tower/Local Control frequency and landing/departing aircraft are advised. In this instance; the B747-400 was still short of the safety area but just slightly beyond runway hold lines on local control frequency. After some research it appears that may have been the incorrect solution. If incorrect then the CRJ should have been sent around until B747-400 was taxied out of the area or could depart. Am still not sure what the correct answer is; since visibility was 20+ miles and a visual approach was being used. If ILS approaches were utilized; it seems cut and dry.