Although I do enjoy reading everyones opinion on poly strings at lower tensions I would like to answer the OP's original question.
You said that your client is looking for "something totally different." i suppose the exact opposite end of rpm at 58 is natural gut at 30. But since that would be a huge disaster for sure make sure to ask specifically what they are looking for. It really comes down to power, control, and durability. Some would add spin into that equation but I like to remind the customer that for the most part spin is something the player creates not the racquet or the string.
After you find out what they're looking to improve ask again and I'm sure everyone here will have plenty of helpful suggestions... not involving why you should string poly at low tensions.
"I knit with fence wire for a living" .... its as exciting as it sounds.