By default Alt + h will bring up the help menu, and alt + i the insert menu. I’m trying to imitate vim-like navigation by setting shortcuts for alt + h = navigate left, alt + j = navigate down etc. This is not possible though, since alt + h will still open up the help menu.
Is it somehow possible to override the menu shortcuts with customized ones? If possible, I’d gladly disable the menu shortcuts altogether 
I’m running LibreOffice 4.2.7.2, now primarily Writer, on Ubuntu 14.04.
You can change all shortcuts to the functions you want.
Tools >Customize > Keyboard
On the top right corner you can even choose if this is for a special application or for LibO in general.
Thanks for the suggestion but it’s not correct though. Menu shortcuts still have higher priority. After a little more research it seems that this is handled by Unity/GTK, so the answer may be to disable gtk mnemonics at a higher level outside LO.
I found out that this problem is probably outside of the scope of LibreOffice, and so I tried to disable the global context shortcuts in Ubuntu, but without luck.
The solution for me was to use xbindkeys to remap the alt + [hjkl] keys to translate to the arrow keys instead. A cool side effect is that I can use that combo in any program, without having to set software specific shortcuts.