Currently you can enclose an element between brackets or bars. You can overline
and underline it too.

But AFAIK there is no way to enclose an element in a box, I mean to draw a
rectagle outline around it. An ugly workaround is to use:

overline underline abs {your_elements}  

or (even uglier):

overline underline left lline {your_elements} right rline

If you're working in Writer, as another workaround which is not always
applicable, you can separate the part you want to enclose in a new formula
object, set a border around it, and then continue you formula in another formula
object. But this is not a good solution, especially when you're working with
very complex formulas, because you have to split your formula and besides that
you have to deal with alignment problems.

I don't know if this feature would requiere any change in the file format.