Being in the firebox to clean or repair it, or maybe the brick arch, while there was still a fire in there?
Thats close enough
To meet the massive demands of WW2 traffic arguably some of the finest and most powerful/efficient steam locomotives ever were created.
These were fitted with all the most modern labour saving gadgets but none could fully clear out a firebox.
The New York Central at the time had a chief who was very pro steam and tried to prove that it could still compete with the 1st gen diesels coming through with regards to availability and turnaround times.
So to prove this point and meet war time traffic demands a job required a pair of workers in full asbestos suits and armed with steam wands to go into the still hot firebox to clean out any remaining clinker.
This practice was officially banned by unions in the UK.
I'd be interested to know if this occurred in the war time era Navy though most US ships had been converted to oil firing.
Your very hot box.