If you wish to hit the desired wavelegnths, you can do it with "royal blue" LEDs. However, they operate at around 50% of the lumens/watt that something like a T5HO bulb does. Semi-defeats the point of the project.
I actually just bought $700 in LEDs and optics last night after thinking about this thread for a while... I'm going to play with aspheric projection lenses.
I wish to be able to mount my LED array flush on the ceiling, have nearly no light directly hitting the glass, and have no annoying glare from the lights when you are in the room. Aspheric projection lenses can make this happen. The downside is cost and labor... I just shelled out $700bones for the chance to just to play with them, and still it doesn't correct the spectrum... Fortunately I'm an LED addict, and I love combining my LED hobby with my aquarium hobby.
Best Wishes,
-Luke