as the user-defined field (UDF) has been assigned at the Project level it will only be available at this level; as you state, the UDF will not be available at the activity level. As regards seeing total for the UDF by WBS, this also will not be possible as the WBS isn't available at the project level.
I would explore using the Funding or possibly the WBS Budget functionality to get what you want; assuming, of course, you aren't using these already.