Why not just write them into the comments field?
1. A lot of extensions lack useful information in this field making it unsuitable for this task.
2. Unrelated extensions may contain the keyword in a different context.
If a  Tags  or  Keyword  field were to be added it would make sense to do a little planning first to
make it efficient and user friendly. Possibly something like this:
Tags: SUBJECT FUNCTION [FUNCTION] [FUNCTION] [FUNCTION]
This way, when doing a search, if there is no match on SUBJECT, you don't any waste time looking at
the other tags. A standard table of  SUBJECTs  and  FUNCTIONs  would make it easier for an end user
to find things and easy for the extension maintainer to pick standard keywords. Example:
SUBJECTs          FUNCTIONs
compiler          c c++ fortran ada cross assembly
driver            audio video tablet network firmware
editor            text hex audio video picture programmers tabbed html office spreadsheet
filemanager       small twopanel dragdrop ftp
fileutil          compress expand sort search backup
game              text graphic network
library           math audio video network fortran
network           monitor fileshare ftp discovery client webserver
player            audio video networkSo if someone is looking to play music over the network, they would search for  player audio network.
If they simply want to know what players are available, they would search for  player, and all players
would be returned by the search. There is an implied  and  in the search, meaning all keywords must
match, as I have my doubts as to how much value would be added by an  or  function.
Although not perfect, I think the above example, while far from complete, has some merit. The difficult
part is coming up with a list of  SUBJECTs  and  FUNCTIONs  that:
1. Everyone can agree upon.
2. That are generic enough so the list of choices does not get out of hand. For example, if you are
    looking for a math library, the only choice for  FUNCTION  is math. No trig, float, integer, etc.
    keywords, as the search will already have narrowed the field to find what you are looking for.
3. That can handle applications that may not fit neatly into one  SUBJECT.