I am a little concerned about injecting slurry at the top of the drum.  This may be hard on the metallurgy above the coke bed if injected at the beginning of a quench cycle while the metal is still hot.

I think there is a  potential of plugging the top of the drum by closing/plugging the channels created when the drum was steam-stripped, and reducing or effecting how the drum cools. Since the top of the drum would not be as hot as the drum inlet, you can end up with mud on-top of your coke bed which can become a safety issue if you get a blow out when un-heading or cutting.