Not only can you edit the league history file, you can also edit each team history file, plus most of the "All Time Records" pages. The game just appends new information each year to the end of these files, so if you add Gold Glove Award info to the leagie and/or team history, then you only have to add the new info each year - the previous info remains saved. You can also rearrange the format somewhat if you want - here are some examples (note the challenge mode scores and the draft pick history in particular).
League history example:
<a href="http://members.shaw.ca/pgroenin/lhistory.htm" target="_blank">http://members.shaw.ca/pgroenin/lhistory.htm</a>
Team History example:
<a href="http://members.shaw.ca/pgroenin/this5.htm" target="_blank">http://members.shaw.ca/pgroenin/this5.htm</a>
If you are in an online league, these files will be overwritten by each new league file you unzip, so you will have to work together with your Commish to make sure the "master" league file is the one that is edited. Also make sure to back up the old versions in case you make an error etc.. One final note, changes to these files are reflected in the game and in the html outputs.