I've been in contact with Klaus before, know his very useful website. There is also the ultravioletphotography forum. Thanks for the pointers re UV LEDs. As far as i understand UV LEDs, they are very narrow in terms of wavelengths, which result essentially in monochromatic (B&W) UV reflectance images. I am not aware of full spectrum UV LEDs (say 320-400 nm). I think I rather have to be careful in re-calculating DOF and step size, and play with distance vs output of flash unit, or find sweet spot with ISO setting. A UV pass IR block filter may be the holy grail, but not aware of one, and at the size I need it would cost a decent fortune. UG11 is blocking visible, but not IR, and then is also attenuating UV. Tried that with the Metz.
The scanner lens is an interesting idea. But first I'll fiddle a bit more with the Coastal.