You might be able to do it by creating a menu item for the form, and setting the "Use Key" to 'number'.
Then in your list settings, set the custom details link to ...
index.php?option=com_fabrik&view=details&formid=X&rowid={yourtable___number}&Itemid=X
Set the formid to your form's numeric ID, and Itemid to the menu item ID you created.
Honesty, I' not sure if this'll work, but give it a go. Basically, the only way to do what you want is to use that "usekey" method, which tells Fabrik to use something other than the PK of your list as the "rowid". You can set that on a form menu, or you can add it on the quest string, like &usekey=number
-- hugh