Apparently yes, Xerox DocuShare (at least 6.6.1) supports CMIS.
Reference: I have found this presentation file written by Xerox which contains this graph:

After talking with a DocuShare expert, it appears that Xerox Mobile Client for DocuShare
is a CMIS client, so Mobile Client Connecter
seems to be a DocuShare-to-CMIS gateway.
The CMIS URL has a pattern like this http://yourserver/<something>/ds_mobile_connector/atom
, with notably the 2 following patterns seen in the wild:
http://yourserver/docushare/ds_mobile_connector/atom
http://yourserver/documents/ds_mobile_connector/atom
Apparently, the CMIS features:
- does not exist in all versions of DocuShare (meaning not on old versions I guess),
- is off by default and has to be enabled manually.