Virtually trying on clothing has significant social hurdles in the fashion industry, primarily due to a complete lack of trust within the industry. Design theft is one of the most major concerns, and any system attempting to enable consumers to virtually try on clothing requires the clothing designs to be released to 3rd parties before the items are in the market - and that early information release is exactly what all social lessons from past interactions in their industry tell them never to allow.

brands can't own their own app (to be fair the gucci on succkks) or LVMH just pools their brands onto a new virtual department store brand?

Personally I don't get the appeal. Seems to me like those who want to actually try on clothes want the tactile feel which you can't get virtually. Versus shoppers like me I just can see from images online what I like and read the fabric, especially a designer I enjoy I already know what the quality is.

... then build a platform that feeds a consumer's measurements to a secure environment controlled by the brand/designer, render the result there, and send the picture back.

Everything can stay under the control of the designer, including 2FA of the end-user, the degree of fidelity of the rendering, rate limits for requests, timed release to selected partners, etc, etc.

Two minutes into that pitch, any and all major fashion corporations will kick you out the door. I spent significant time trying to crack this nut - the fashion industry culture at it's core is luddite, that and any tech that is not 100% transparent ain't gonna fly.