"even optical glass is pretty much opaque to it." Which would make it a bloody awful defence- it'd be absorbing all that UV light, heat up and crack.
The problem here is they're thinking about it as a single beam. They'd be better off with a number of slightly less impressively-sized lasers. Like a truck-back-full of 10W diodes, all firing up fibres. These fibres would then be aimed so that the emergent laser light was concentrated on a single spot (i.e. on the shell, which the on-board sensors would track in all 3 dimensions) using a bucketload of servos or some other type of actuator.
This would have the benefit of saving anyone stood in front of them (troops fixing it in the event of firing) or behind them (big buildings, planes). Also means you'd need a bloody huge projectile to make a significant dent in the power of it as so many elements were actually doing the firing. A 10W laser is only a couple of cm x a couple of cm x a couple of cm, and a truck is a couple of meters each direction. So 1000 elements wouldn't be horrendously hard, and 10000 wouldn't seem impossible. Even with the other equipment (radar, cooling, etc).
Assume that Boeing have access to more powerful semiconductor lasers than you or I, and suddenly it all becomes very possible. And the lack of any high-power light anywhere except the focus point means you've not got any problems of melting mirrors or lenses.
Another advantage would be that due to the low mass actually being shifted per fibre-end, you'd be able to target it far more quickly and efficiently than you could with a massive monolithic laser. You could concievably even scan CRT-style over the area the enemy shells are passing through, illuminating them and focussing for a few seconds on anything that's reflective.
Plus with power and targetting like that you could probably create 3D holograms like this (http://blog.intuitymedialab.eu/2008/04/30/three-dimensional-images-in-the-air/) with a decent resolution. Just imagine being some random islamic terrorist in Iraq seeing some ethereal image of Allah, God or an Angel appearing in the sky. Or seeing "good guy"/"western" troops being preceeded by glowing galloping horses who tear through the terrorists' friends. You'd probably need more elements to manage a decent resolution, refresh rate and power rating, but this is Boeing working for the US military- it's not like they'll have a lack of trucks or funds!
That, and you could put on a helluva light show at the end of the night...