Why do you need to use a linked table to access the dbf with the SHP? What type of data source connection are you using to access the dbf as a secondary source? I'm not sure it's possible to connect to the DBF as a secondary source if it is in the same folder as the SHP since the SHP provider will put a lock on the DBF when it opens the SHP file. You can't separate the DBF from the SHP either.
As far as I know there is nothing that should prevent special characters from a secondary source, but testing with an SHP that has french characters in the DBF work fine both in tooltips and labels.
You may have to use the SDF Loader to convert the SHP to SDF and then link to the table. Is the data static or updated periodically?
Regards,
Dave