I don't have an answer but I had the exact same question.
See page 105 for applicable environment variables for IPS.
PD22894 - Host Intrusion Prevention 8.0 for ePO 4.5 Product Guide
I was able to get variables to resolve using the %VARIABLE% syntax, however, every time it resolved the directory, it left out the backslashes. E.g., %SYSTEMROOT% resolves to C:WINDOWS, instead of C:\WINDOWS. %USERPROFILE% (for SYSTEM account) resolves to C:WINDOWSSYSTEM32CONFIGSYSTEMPROFILE, instead of C:\WINDOWS\SYSTEM32\CONFIG\SYSTEMPROFILE. I'm not sure why this is though.
The [iEnv SystemRoot] does not appear to be the correct syntax for HIPS 8. I'll have to test this further. I would suggest contacting McAfee Support to formally open a Service Request.