I don't know that specific tool, but generally any third party can read from the Rational Data Warehouse if it wants.  We're doing an integration with some Oracle-based applications right now, and it will likely involve mapping the Rational DW stuff to similar things in the foreign database with Cognos as the reporting tool.  As long as you don't start messing up existing tables and such in the Rational Data Warehouse, you can add more tables and views and functions and whatever you need to it.  That's part of Insight customization but it should work with anything else.

Your Tableau people will probably have to develop a data model of some sort to map the data warehouse attributes to useful/readable things; again, not familiar with that application so I don't know the specifics.  You could also ETL the data you need from the data warehouse.  Writing your own ETLs for CLM from the ground up instead of using whatever DCC gathers would be a whole bunch of extra work; if you cannot just map to Rational Data Warehouse and need to ETL into some other warehouse for some reason, just extract data from the data warehouse.

These data dictionaries will be useful to your data modelers:

https://www-01.ibm.com/support/knowledgecenter/api/content/nl/en-us/SSCP65_5.0.2/com.ibm.rational.reporting.overview.doc/topics/c_reference_datadictionary.html