Having only recently caught up with the Sketchbooks, I was pleased to see that this piece is in Sketchbook #5.
I never found out if Stan drew this specifically for me, or if he happened to already have a drawing that matched my commission request (made prior to MegaCon 2008). I had asked for a happy father/son moment between Usagi and Jotaro, like the charming piece in this thread:
http://www.usagiyojimbo.com/forum/viewtopic.php?t=3102
IIRC, the drawing on the page facing this in the Sketchbook was also a commission someone posted here.