a supply for a week would fill most small waist sized bags
Well, yes, but surely that's the very point ......
In the event of an evacuation, you do not need to take a week's supply of medication with you !
I read what you said above about it being expensive and "rare" medication, but in this day and age, its not beyond the whit of man to courier you something, either domestically or internationally ... and for that item to be with you the same day or next day at the latest. Your travel insurance would no doubt help take care of organising it and pay the associated costs !
I'm afraid I'm with the others here who have said that I don't give a toss what you are fetching out of the overhead locker ... if you're blocking my way during an evacuation, I'm not going to be politely standing there waiting for you to finish rummaging about !
At the very least you should consider just putting your essential de-minimis medications into a plastic bag and sticking it in the seat pocket infront of you so that you can instantly grab it without any faffing about.