Simply put, with the Rapid7 App for Splunk Enterprise app work on Splunk 5.0.5?
The Rapid7 application utilises the Splunk Web Framework. This is integrated into Splunk 6 only but is available in a stand-alone version for Splunk 5. Details here.
Officially the Rapid7 Application for Splunk only supports Splunk 6.0 and higher but assuming the standalone Web Framework is installed in Splunk 5, the application should work.
Short answer: No. Long Answer: Yes (but unofficially and without support).