Hi,
Through the Hierarchical Menu from SCN Community Package, I have been able to provide this option.
I thought people would migrate quickly to 1.6, and didn't take the time to create it for 1.5. Here's the blog entry for 1.6.
That being said, my first working prototype was on 1.5 and it worked fine.
If you want, I could improve the SCN Com. Package 2.0 (for 1.5) with it. It would take a couple of days though.
If you know how to write SDK extensions, you could look at my code and adapt it to 1.5 (I could give you insight about the algorithm as well).
You could also use the first version of the hierarchical menu from the 2.0 SCN Community Package and try to build your grid layout. I am pretty sure it feasible since new CSS trends are using list and "display:grid" to emulate this behaviour. Here's the documentation.
Let me know.