I completely understand your apprehension.
As an advisor to some virtual teams, this can sometimes stem from distrust of procedures and in other cases, it can be an 'order from upstairs' given in the name of 'quality control'.

Short answer - This cannot become standard protocol. It will cost you time, employee trust (they will feel let down by you) and give clients a sense of power that is unwarranted and may be misused.

That said - you must continue a conversation with the client on your level. Is the project high risk? Have they had a history of bad fits before? They have a right to be heard regardless, given reassurances to specific grievances, and exceptions can be made, but your employees need and expect your protection first.

Happy to continue the conversation!