We considered adding a little sink to our 25 head as well but just could not find the space. I was planning to build a small unit that would fold upward against the wall when not in use and it just didn't pan out to be important enough. That pic is a Tomcat I believe, which has more space in the head than a 25, so what looks reasonable for the tomcat does not fit the same on your boat. If you or your lady friend have used the head as a shower, imagine a space that you didn't need to get through the event. Like Bob, I did find it roomy enough to share with much. Most of the dead space in the head is down at floor level.
Good luck and make some cardboard test versions.
Thanks Greg.
Greg