Can you change the loading screen tips? I've tried changing it through a hooked strings_db locale file, but it doesn't work 
which is strange, since that same file can be used to change other things like unit names and descriptions.
I've also tried changing /lua/ui/help/loadingtips.lua by putting it in /hook/lua to no avail.
Any ideas?
Loading screen is displayed before any mod is loaded, so you can't hook it with a mod.
Why do you need to change the loadingtips localy on your machine ?
in case you need to test something you can modify the file
C:\ProgramData\FAForever\gamedata\lua.nx2\ui\help\loadingtips.lua
To see the effect you also need to modify your language file.
(basically you only need to change the language file)
C:\ProgramData\FAForever\gamedata\loc.nx2\US\strings_db.lua
.nx2 is only a zip archive.
I would want to integrate it into a mod-but that does make sense if it can't be hooked, because it's being displayed while the mod is being loaded.
Thanks!
have in mind you always can distribute new loadingtips to the game itself.
Just write an issue or make an PR on Github:
https://github.com/FAForever/fa
Depending on your effort and the quality of your tips it would be also possible to patch the game
and make it possible to load tips from a mod before the loading screen is displayed.
(i would not like that, good tips should be available from the default game)
Last "and" least you can make a .SCD mod. That means you make an archive with the changed files and put it direcly as *.scd file into the gamedata directory. This will overwrite the original files before loading the game.
But i would not recommend that. Since it will overwrite the original files it must be fixed after every game patch.
Including your tips to the game itself (Git-Hub) would be the best option!