Dear Chris,
It is possible to post Parked document via FBV0.
Can you please elaborate what error you have got while posting.
Also, Please check whether you have created any workflow variant and attached to your company code.
Check your customization IMG->Finance->Document->Parking Document.
In transaction Code OBWA, check the indicator "Posting Release".
If the indicator is set, you cannot post the document directly in FBV0.
It has to be posted via work item functionality.
And for this reason, the posting button in FBV0 is inactivated by the system.
You can find out from the list of parked documents whether workflow was triggered or not at the time of parking the document.
FBV0 MENU->Document->List
To check whether workflow has been triggered in the parked document.
If "W" has mark "X", it means workflow is triggered and it has to be released before posting.
Hope this information helps.
Regards
Rajesh