Depending on the data source you are using, you may be able to use Custom SQL to do this. Connecting to Excel, for example, you can use the legacy connector (the jet driver you mentioned above) and you can do something like this:
TRIM is supported on that driver so you can define a trimmed column for each data source, then use it to make the join. The filter section on either the data source window or the sheets is still available!
Also, a BETA version of Tableau 10.2 was just launched and one of the features Coming Soon is called 'join calculations'. That may be useful as well! You can join the BETA program in that same page.
Hope this helps!