There's the configuration variable in the output profile to add 'just' un-supported letters there - I don't think it would be a great idea to add letters to the defaults because users taking the time to add fonts to their readers would complain, but maybe a 'tweak' could added so users could append their own list of characters.
@millhaus, I don't know if you've checked on the Sony sub-forums, but I thought that you could just stick a font file onto the Sony's filesystem directly and use Calibre's extra_css to have every ebook use it once converted. I might be mis-remembering though and it's possible that it only works with the latest generation devices.
Couldn't find it on Mobilread, though I'm sure the info is here somewhere, anyway here's a link to the regular Sony forum:
http://www.sonyreaderboards.com/foru...le=6&f=2&t=190