If you have chat or any text field highlighted, Tab will still open the radial menu.
It's probably not a good idea to remove this since you want the radial menu accessible from everywhere.
Problem is that Tab is also used for auto completing player names/admin commands/... Every time you use it, you see the beginning of the radial menu pop up.
And moving the auto complete button to somewhere else is probably not a good idea either, auto complete is set to Tab in pretty much every other program.
A possible solution would be to require you to hold the Tab button for 300-500ms before it pops up, not sure if that would be noticeable/annoying though.