Probably okay the corn head grease flows pretty well as it warms up but I don't think you can get enough in from the top to push all the way down to the bottom.  I like to grease from the bottom zerks though & remove top plug and set a golf ball over the opening when the grease pushes it off it's full enough. Lock and lube on a battery grease gun makes it go pretty fast. Be sure and use the header stop for safety. I like to top them off every year as there are coil springs that deliver grease up to the top sprocket bushings.