my initial thoughts are:
already products on the market for this (inc bebe au lait as you say)
you tend to draw more attention to yourself putting on some bizarre garment than you do just sitting and feeding
not sure that it's hugely different than wearing a vest top and a normal top. pull vest down and regular top up...
the marketing of such things almost implies that there is a need to use them ie: that you shouldn't be feeding openly.
i do also agree though that if it helps someone feed in public where they wouldn't hav e done otherwise then that's good
i'll share a quick experience of mine while I'm here thouhg
M&S cafe. bit "upmarket" round here lol, but my cafe of choice
sat a few tables away from another lady with a small baby. my baby starts to root around, so I whip out boob and placate him. carry on eating my lunch, all good.
baby down the way starts to grizzle. Mum pulls out this big bright green shawl, starts to wrap it round herself, has her head under it (and baby)... I am not kidding you, everyone was looking at her. baby getting more and more vocal cue more attention on her. She finally emerges from under this thing looking all red and flustered. sits and feeds
I just think they're so unnecessary!!! I don't think a single person noticed me feeding ds2 in that cafe, and if they had well.. that's ok!
I'd rather we spent our time and energy normalising breastfeeding than inventing ways to stop other people seeing it tbh