Using RTC with DB2's HADR feature, need purchase DB2 license for stanby server?

 This needs to be checked with DB2 forum. Jazz/RTC can not answer this question.

Your RTC entitlement to DB2 is covered under "Optional Programs" section of the license (for 4.0.3, see: https://jazz.net/downloads/rational-team-concert/releases/4.0.3?p=optionalPrograms ).
This in turns refers you to the licensing page (for 4.0.3, see: https://jazz.net/downloads/rational-team-concert/releases/4.0.3?p=license/license_en ).

The two terms that appear relevant here are: 
"Each valid client license includes a license to run one instance of DB2 Workgroup or Enterprise Edition"
and
"Licensee is authorized to install and use such Supporting Programs only to support Licensee's use of the Principal Program under this Agreement and within the limits of the Proofs of Entitlement for the Program".
So I'd conclude from this that you are allowed to use multiple DB2 instances (one for each client license you purchased), as long as they are all being used to support RTC and not some other DB2 application.  You would need to confirm that DB2 Enterprise Edition provides the HADR capabilities you are looking for.

Note: I've sent email to the RTC product manager to verify that my interpretation is correct.