ECM isn't the only source of unexpected high power RF. As a radio ham, I'm allowed to run a few hundred watts on a wide selection of bands from shortwave to microwave, and to do it in a large number of interesting ways - including mobile. There are health and safety rules about RF exposure which rule out being too antisocial, but I - and I guess most hams - are a bit hazy about those and wouldn't know how to apply them properly. They're certainly not part of the licence requirements, nor are we expected to demonstrate knowledge or practical abilities in that area. (The implications of this are probably best left for another forum!).
The upshot of this is that equipment designers and regulators should not - as far as I'm aware, do not - assume that the RF environment outside an aircraft is uniformly benign. Even if they don't think it likely that a bunch of radio nuts will be setting up a moonbounce experiment beneath the approach at LHR, it's worth thinking about the security implications of having stuff vulnerable to attacks from hostile elements with the nous to knock up some form of directed energy weapon. Fortunately, the physics of airframes provides a very good first line of defence. I doubt any Prime Ministerial ECM that wasn't going around knocking out car electronics on the road around the convoy would trouble a 777 - and in any case, that sort of ECM isn't there to make ordinary electronics malfunction.
I'm more interested in finding ways to work out whether consumer electronics does cause problems, as gathering evidence for this ahead of a serious accident will be the only way to influence the industry to work towards a solution - which, as I've said, will probably be some sort of self-certifying regulatory regime.
R