Sorry for the intrusion to our Sapporo locals and not sure about all you can eat options:

In Susukino ( the entertainment district of Sapporo ) the Kani Shogun ( http://www.kani-ya.co.jp/ ). But as Sapporo is landlocked the freshest crab is to be had at Otaru a short train ride away. There are several excellent restaurants serving crab dishes at the Otaru Unga buildings along the canal.

Maybe the Sapporo Brewery restaurant does an all you can eat buffet?

As in many cases, Otaru is actually the older town of the two and used to have a bigger population for a long period before the pre-eminence of Sapporo. Darn I miss Hokkaido...