I had the same dillema a two weeks ago... Got the same answer from recaro...
I got Recaro START Seat for my daughter. No matter what the suggestions are I fixed the seat using the holes on the seat to X3 ISOFIX anchors using plastic wire clip.

Now I have the seat which is fixed for city driving when my daughter is not with us (I dont have to put the belt when the seat is unused) and it will not move arround the car.

When the seat is occupied I am using it with 3-point tensioner...

Regards

Alex