If any of the tools you mention were readily doable today, I fear we'd be so deep in trouble, no canary would help.
Seriously, most online security works only because the competent people have better things to do.
I wasted _days_ recently trying to track down code bugs that weren't there - a piece of Cisco gear that was in the clients network was running a standard configuration called SMTP Fixup which was deep packet inspecting and rewriting the "250-STARTTLS" capability responses and passing them on as "250-XXXXXXXA" on the fly. It took me way longer than it should have to debug, partly 'cause I started looking in the wrong place, but largely because most of the testing we did was with mail clients that were perfectly happy to transfer mail unencrypted when the STARTTLS capability wasn't announced.
Anybody MITMing you in Starbucks could easily do the same.
A little bit of thinking with my "evil hat" on leads me to believe a similar protocol aware packet inspection/modification tool could easily rewrite webpages on the fly, looking for links to common service login forms and rewrite appropriate links and form actions to be http instead of https...
Maybe an appropriately paranoid way to set up this sort of canary is to have all your mobile (ie, non-fixed ip address) devices use a vpn into a trusted and well secured host?
That already exists in the form of sslstrip: http://www.thoughtcrime.org/software/sslstrip/