Learned today that SBS still doesn't support DirSync. You can install it on a domain controller which didn't used to be the case. So the requirement remains 2008/2012, including DCs. No SBS 2011 (what I was trying in this case).
I haven't seen any information to suggest anyone has gotten it to work. Feel free to share your experience if you have. My assumption is that SBS being a different beast with its extra SQL Express and such just can't do it.