Excel allows the user to choose whether to save the file as PDF/A in the options of the save dialog. This setting is stored in the Registry as LastISO19005-1 (REG_DWORD) under HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Common\FixedFormat. (Replace 12.0 with the right version.) The ExportAsFixedFormat function also respects this setting. You can set this value to 1 before calling ExportAsFixedFormat to get Excel to export the file as PDF/A.
I know this solution is not pretty as it uses global state to address a local problem. Consider restoring the previous value when you're done.