You could kind of sort of do it using custom characters to create a "large" font.
You would need to create the blocks for the larger font but then invert them.
If all you need is digits then the custom chars can be pretty simple.
If you don't already have a character that is "all pixels on" reserve 1 custom character to be all "black".
You would end up with a mostly dark display but there would still be
light around the edges and in between the LCD characters.
There are a few example sketches floating around out there that do large fonts on character LCDs
and here is thread that talks about it:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1245352653
In your case you will want to invert all the bits to allow the light to show through.
--- bill