Now I see you have to fit in with the Editor of Vogue . You are right that repurposing a designer messenger bag would be best . You would not have to walk far from the Louve to see almost every bag worth considering
. The leather on the Montblanc line is what I would look for . Look at the Overgaard blog posts one bags . He differentiates between the travel bag size and the carry size . No way to make a travel size bag look elegant on a carry .
I found its best to have the exact dimensions available . For example ...a bag that is thin about 3-4 inches carries best . If the width can be as needed (no bottom piece ) even better . Second height can really help look into stacking lenses verse side by side . (remember the camera and one lens will be out most of the time ). Third consider that you may want to carry the body without a lens . This allows for the height of the body to go along the bag length .
Having a good handle on the dimensions can make finding your bag easier . I like a bag that's length(across) 10-12 ,height 7-9,width (depth ) 3-4.
I would use Leica M bodies and no more than three lenses. However..... I fully understand the value of using the larger sensor and having the MF aesthetic and I can not say your approach isn t best . Makes me want to dust off my ancient HB 500c/m and shoot film . No bag just standard lens .
The other thing that's nice is to have an Apple iPad MINI remotely tethered (sort of ) similar to Leica Fotos application .