Authenticode signature of the process' main EXE file and all its DLLs can be verified using PKIBlackbox package of our SecureBlackbox product.
Such check should be performed from OnOpenFileC/OnCreateFileC, but to improve performance you need to cache validation results. For example, you need to calculate CRC32 of the EXE file and remember it together with validation result. On next checks you don't perform full validation but only compare the CRC32. If the CRC differs, then you don't need full certificate re-validation.