Hi
Requests for last minute high rates of descent are not common operational practice, however on occasion's they are unavoidable. If I find myself in this situation, I would normally warn the pilot that after the conflicting traffic clears I will require a high rate of descent, just to get the crew ready for the executive instruction. Most importantly, if the 'level by' is unachievable then inform ATC, as hopefully plan B will be implemented.
Good luck
GMWTB