I've requested a similar feature some time ago - basically the ability to hit a hotkey to select all tracks within the same folder as the currently selected file(s). mp3tag is great software and would be even better with some usability tweaks like this. It would be nice to see some focus on this even if at the expense of supporting yet another audio format or itunes tag.
Using the mouse is an option, but you have to position the cursor in the right spot for the action to happen and it's just as susceptible to over/undershooting as the keyboard method. A hotkey is an elegant solution and can even be made to select all files in the next folder when pressed a second time.
Here's the thread covering my request: /t/8577/1