I'm aware of how those workarounds, but this is suggestion is for something simpler and more visible sitewide.
@Martok yes, that's what I do, ie I put in a big notice but, you don't see that in the ACP. Editing the test site title would not be very obvious. So I suggest something right across the top on all pages inc ACP
Too turn it on I'm just thinking of another tick box in Options > Board Active e.g. Board Test Mode Notice