Steve,
whilst I have no experience in applyin an protector to leather, I would be extreamly hesitant to do so.
A solvent based product, even if sake on the dye would most likely have an astringent effect on the leather, which could cause cracking/splits/delamination over a period of months, At the very least it would reverse the affect of the conditioning oils so important to it's continued good health.
A water based product would largely run off a properly cleaned suite due to the conditioning treatment.
I have come across some waxes (mainly for shoes) which perport to contain Scotchguard but the lack of general adoption by 'Cherry Blossom' etc. sugests that the addative was more for marketing puposes than any real-world value.
Sorry it's not an authorative answer - more a cautionary note and likely echos thoughts of your own but I would walk away from the job if the customer remains insistant.
John.