Cool idea!
I'd love to see a square aps-c based sensor, with the lens projecting in a circle juuuust big enough to cover the edges of the sensor to achieve the same vignette as a film holga. lens must be low contrast, with high resolution center and a fast dropoff to the edges. Make the sensor reeeaaally reflective so you get weird flares and low contrast spots etc..
Figured I'd show you guys this.
They released a 25mm f/8 Holga Lens for M 4/3. (50mm Equivalent)
The Digital Pen series can shoot in square format. Anywho, the output is nearly impossible to tell the difference from a film Holga.
Can buy them here: http://cgi.ebay.com/HOLGA-HLW-OP-wi...era_Lenses&hash=item3cb76fe89b#ht_1908wt_1141