We've seen a lot of interest in running PostGIS on Postgres-XC recently, but unfortunately, setting things up is not as straightforward as it is in PostgreSQL. In PostgreSQL, you simple enter:
CREATE EXTENSION postgis;
Postgres-XC does support extensions, but PostGIS is not your average extension. There are a number of dependencies as well as the spatial_ref_sys table that needs to be properly created on all of the data-nodes. To help people get up and going with PostGIS on Postgres-XC, we've create a tutorial on how to set it up.