I played around with it a bit, but I could not get the multi file upload to work in ad mode. I was able to get it to auto save the record and do the multi file upload, except the user then was left on the end of the multi file uploadscreen, and there is no way for the user to get back to the saved parent.
I guess the best I could suggest would be to put a button on the form which would be called something like [Show multi file upload button]. That field would only show in add mode and would be located right beside the native Plug In Multi file upload button - the latter being hidden in Add Mocde). What that field would actually do would be to save the record, and land the user in View mode and they would now see use the multi file upload button which you have set to show in view mode or in edit mode.