I found some wakeup radio activity.  Some of the recent stuff looks almost usable, but of course I don’t understand the electronics, so it might all be bogus..
Very low power.
It all seems to be still in the lab. AFAIK there’s nothing that can be bought today to mount on a board. It has to be coming soon though.


I saw this, and it looked deceptively simple.
Inline image 4
and a few kindergarten sums that we both totally understand
Inline image 2
I poked around a bit more and found this chip, and a 2015 datasheet, and some more description AS3932.  
and this paper which contained this interesting table

One of the very few commercially available is a low-power, low-frequency wake-up receiver chip with addressing capability AS3932 [14]. Gamm et al. [15] designed a low-power WURx circuitry around it. The main transceiver produces a 125 kHz wake-up signal OOK (On Off Keying) modulated on an 868 MHz carrier. They report communication range of 45 m when transmitting with main high-consuming transceiver with +11 dBm and over 15 m when using 0 dBm output power.

There is a demo kit 
Then I tripped over this paper 

6. Conclusions and Future WorkWake-up Radio (WuR) systems provide significant energy savings for wireless sensors when compared to conventional duty-cycling approaches. In this paper, we investigated and characterized a promising novel WuR approach that is based on SubCarrier Modulation (SCM), which enables two radio operation modes. When the remote sensor node is in low-power wake-up mode, a Wake-up Call (WuC) can trigger it remotely. Afterwards, the node switches to data communication mode to start the data exchange, e.g., wirelessly reply a transducer measure back. In this paper, we analyzed the performance results of the SCM-WuR approach and compared them to state-of-the-art WuR systems as of 2013. We conveyed the detailed performance analysis of the SCM-WuR approach in terms of wake-up delay, current consumption and overall operational range. Through physical tests, measurements and simulations, we have shown that SCM-WuR systems feature an outstanding tradeoff between hardware complexity, current consumption and operational range. We also demonstrated how SCM-WuR systems enable multi-hop wake-up for even longer remote sensor measure collection. For this purpose, we implemented a wake-up relay node and characterized its operation through delay and current consumption analysis. Finally, we have presented two real sensor monitoring application cases with SCM-WuR system, one single-hop and one multi-hop scenario, and calculated the lifetime and packet delivery ratio of the latter through simulations. Indeed, SCM-WuR systems effectively enable a vast range of applications, while most state-of-the-art proposals are restricted to short and medium-range scenarios. To the best of authors’ knowledge, this study is the first one to perform such type of global analysis in the field of Wake-up Radio systems. As future work, we target the evaluation of SCM-WuR design for different frequency bands, a feature that allows its integration into different wireless technologies. 

which had the following interesting chart.