Hi Steve,

At this point, there's no good ("official and supported") way for 3rd party software to collect this information.   

However, Reliability Analysis Component (RAC), the data provider for Reliability Monitor, does publish an event to the eventlog that contains the stability index for that 24 hour session.  The details of the event are as follows:

Log name: Microsoft-Windows-Reliability-Analysis-Engine/Operational

Event ID: 2005

A full, official solution is being considered for future versions of RAC.

If you have questions, feel free to email me at valeraz@microsoft.com

Cheers,
-Valera