Pretty straight forward
Energy absorption required per cubic meter of air. Absorption co-efficient; Single point source inefficiencies in heating the 'fat' and 'thin' parts of the lens. Perhaps you could use the heat generation from the overworked multiple CPU's and IR lasers with some form of instantaneous heat transmission. Then there would be the small problem of H2O absorption and re-radiation. And the small technical problem of the Zeppelin carrying the nuclear reactor, and/or the very small problem of a fixed wing aircraft having a minor issue with retaining state as it's energy source was diverted (perhaps a 10^5 tonne aircraft in a rapidly decreasing altitude scenario could fuel this for a few seconds); and BTW how would you know where to look for the threats if you hadn't already used this?
Apart from that seems quite sensible to me.
BTW, has anyone in BAE studied the views of Charlie Stross - if we could only get the Cthulu stuff working it might be cheaper.