Thanks for the replies.
Unfortunately, it looks like the Accell Mini DisplayPort 1.2 to HDMI 2.0 Active Adapter is vaporware.
There are certainly a lot of Mini DisplayPort to HDMI 1.4 adapters out there, most of which obfuscate their capabilities by calling the output "4K HDMI", which isn't even a spec, but means 4K at 30 Hz. Apple itself is guilty of the "4K HDMI" obfuscation.
As for the Thunderbolt docks, it's the same story. HDMI specs for some TB docks:
OWC Thunderbolt2 Dock (HDMI 1.4b)
Elgato Thunderbolt 2 Dock (HDMI 1.4b)
Kanex Thunderbolt 2 Express Dock (HDMI 1.4b)
Belkin Thunderbolt 2 Express HD Dock (no spec for HDMI port)
StarTech Thunderbolt 2 4K Dock (no spec for HDMI port)
IOGear Thunderbolt 2 Dock (no spec for HDMI port)
Sonnet Echo 15+ Thunderbolt 2 Dock (no HDMI port)
AKiTiO Thunder2 Dock (no HDMI port)
What am I missing here - am I the only guy who wants to use a 4K TV as a monitor?