Weight out front makes the handling not great but stuff is moveable and can still keep some speed. Weight above handlebar height is a lot more sketchy, if you hit a bump and the front wheel comes off the ground the bike starts tipping over in the air, so you need to go quite slow.
How does the omnium cope with weighty cargo up front? do they not get quite heavy as soon as the weight is off centre at that elevation?
with the rack on my fixed bike loaded with some compost it loves to nose bang the tarmac!