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Jeff,
In principle, this should work, but as usual, the devil is in the details. The most obvious to me is that you are limited to small boards with this set up. The glass has to be larger than your original, and it also has to have a flat field, so as not to introduce distortion. Large glass with good optical characteristic tends to be expensive. The glass will also have to be of a type that has a high transmission of UV if you are exposing photoresist. You will have to provide a mechanism to position the lamp/lens combination to optimize results, and also a mechanism for focusing the image on the board. Focus could also be an issue if going to photoresist. The focal point for UV light will be a bit different from white light because of the shorter wavelength. Somewhere in the setup, there has to be a structure to place the lens the right distance from the board to produce the exact magnification. This would be an awesome project...
I am not sure what problem this solves. The major problem with laser printing that I see is that the narrower the trace, the fewer dots there will be to generate the trace width. With a 600 dpi printer, a 10 mil trace that is aligned with the printer scan direction will have 6 dots. This is relatively reasonable. A 5 mil trace with 3 dots is not. Changing to a 1200 dpi printer solves that problem, but at expense. Clearly, the transfer process is limited in trace size by the resolution of the printer. Printing on transparencies, and using them as photomasks with the photoresist process has the same limitation because we are still dealing with too few dots. To do this properly requires a high quality laser plotter to make the photo mask, or in direct write to the photoresist.
A setup for a reducing lens -http://archive.cnx.org/resources/fb001372e360c7bddb6939364bfb77a57420e7dd/5.15.png