I know for a fact it is possible. I am acquainted with veterinaries and have kept chickens temporarily while they were recuperating.

> They tend to hide any illness until very sick.

Indeed. So do rabbits and other “exotic” pets. It does make treatment harder, but experienced people tend to develop a sense to notice it sooner. You yourself have probably already developed that skill to some extent and might be able to identify “strange” behaviour is specific individual chickens.