You could always just simulate it by having the plane fly over but not necissarily drop the bomb, by creating the waypoint set to move on your alpha radio and then a trigger over the building you want bombed with the following in the condition field,
harrier in thislist
and in the on activation field i believe it is something along the lines of
harrierbomb = "Bo_GBU12_LGB" createvehicle (getpost triggername)
this wont force the harrier to drop its bomb, but will simulate the bomb landing on the target, if you wish to be able to see the bomb drop, use
harrierbomb = "Bo_GBU12_LGB" createvehicle [getpos trigger select 0, getpost trigger select 1, 100]
100 is equal to the height the bomb drops from so can be changed to look like it drops from the harrier.