Here's a workaround for getting the label on a grand staff to appear in the correct place - i.e. at the middle of the brace. This requires version 1.75.

The first step is to make the labels for the upper and lower halves of the grand staff invisible. Do this by naming them with strings of the Alt+0160 character. This is a non breaking space - it's a space which behaves as a character, and not just a gap. (Staves are named in Staff | Staff Properties | General.) Name the top one Alt+0160, and the second Alt+0160 Alt+0160.

Next, create a new staff between the upper and lower halves of the grand staff. In Properties, set the upper and lower vertical size to 1, and the number of lines to 0. Name this staff "Piano". Fill it (to the end of the piece) with invisible whole bar rests and visible barlines. Voilà!