You should be able to do a records request to Onstar or subpoena the records. Since you are attempting to gather data about your own car, you should be able to request the data after showing proof of ownership. Id suggest calling Onstar.
Here is what I found on their legal compliance page:
HOW TO CONTACT US
If you have concerns or questions regarding GM's consumer privacy practices or this Privacy Statement, please press the blue button in your vehicle, visit our website(s), or call us at 1-866-MYPRIVACY (1-866-697-7482) or 1-888-4ONSTAR (1-888-466-7827) for OnStar questions. You may also contact the Privacy Manager by mail at OnStar Subscriber Services, P.O. Box 1027, Warren, Michigan USA 48090-1027 or by email at
privacy@gm.com.
Here is what I found on another legal compliance page:
Address:
ONSTAR CENTER, P.O. BOX 0217
TROY
MI
48099
Phone:
(888) 466-7828
I cannot promise any of this is accurate.