I have a box and a text over it. They are grouped, and they are aligned such that their centers are in the same point. How can I make inkscape realign them automatically when I change text, so that it's center remains in the box's center?

There's not really a way to do that fully. However, you can get kind of close. When you create the text object, if you set its alignment to center, when you add or remove letters, the text will stay centered around its current center.

Flowing the text into the box as a frame won't work because it will only respect horizontal alignment, not vertical.

Thanks, this is exactly what I need, but I didn't understand: how do I set alignment to center?

When you're using the text tool , the toolbar will change to show text-related options, like font, size, alignment, bold, etc. There are 4 alignment buttons: left, centered, right, and justified. You can also change the alignment from the Text and Font dialog (Shift-Ctrl-T).

thanks!