To access the numpad keys I use the left k-shift, but I am not using the 2nd layer just for the numpad. I mapped all the F keys to the number row, so for F1 - F5 I hold the right k-shift and the corresponding number key on the number row. Also, since I normally map End to Escape, and I sometimes need the End function, I use the right k-shift and the End key to produce End. By the way, I use the embedded numpad for mousekeys. I just hold the left k-shift and can do clicks and move the pointer right around the home row!
I'm thinking to add a couple more keys, around the thumb clusters, so I can do shift with my thumbs.