The form has been closed.
I came up with a work around - editing the custom action in the IPFS file:
%PROGRAMFILES%\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\FEATURES\IPFSSiteFeatures\FormServerEcbEntry\EcbEntry.xml
...and changing the appropriate UrlAction elements to include a source parameter, i.e.
It's not the best solution however (because the file will get overwritten by any updates etc.) - but I haven't seen any other answers out there.