Even though I do not have the whole picture, I’d not consider going back to that drum after 10 hrs; I’d have considered withim couple hours, but after that long it may be worse building more coke on top of an unstable coke bed.
Now, unheading that drum will be always a risk and safety / fire protection meassures should extreme as much as possible. Putting steam in a quasi empty drum is not a guarantee of adequate stripping.
Few questions, did the drum receive cold feed? how long? what’s the type of unheading device? if Slide Valve available, then putting water and try draining would be my first attemp; if drum won’t drain, let it soak and open the slide valve being prepared for anything. The worse scenario is if either manual or semi-automatic unheading is available, because it would require special facilities for potential hot flamable material release.