When it looked, briefly, like there were going to be two separate letter spreadsheets I had a little fiddle with importing both into a database where you could organise letters either by username or by fandom.
The problem seems to have largely gone away now but, on the off chance the database will be useful and since it's been built I thought I'd let people know where it is:
Yuletide Letters Database.
I'll update it from the spreadsheet roughly once a day.
It should be noted that the database and interface is very basic and I've not (yet) figured out how to get non-roman characters into the database. I'll keep fiddling with it so if there is anything particular people want let me know (though be aware my time is finite).
If there are coders out there who'd like to play with it/help then all the code is in
a github repository.
EDIT: For reference,
this is the current post for submitting letters to the spreadsheet. I just use the data compiled by the spreadsheeting team who are really doing all the heavy lifting here.