Holding a coveted bag for an unknown, who may or may not purchase, would be a untenable practice for a premier house whose goal is to keep stock for regular or walk in clientele.
Speaking from my very limited experience as a walk in tourist to NBS and CP, it is only once you are there in person that you can click with an SA who will then assess your needs. I cannot imagine doing more than one location in just a few hours before getting on a train. When I went to CP, I was actually looking for RTW for DH that we missed or was not available at home (CP didn’t have what we wanted, but we did get a sweater at NBS).
At CP, I made it very clear that we mainly shop in our local store, but we got into conversation with an SA about a particular ALange watch. He later offered to show us an etoupe Chevre geta, PHW, that was available for purchase. I looked at the geta out of curiosity, but I politely declined, as I am more a mini Della cavalleria person. Regardless of my particular offer/outcome, I doubt that I could have called to inquire about x item, even the offered geta, bc Hermes culture doesn’t operate that way.
ETA: while I was fortunate enough to obtain an FSH leather appt and a K25, it is clear from the Paris thread that lottery is increasingly hard to get. For many, your local SA is the easiest bet.