You could use an INSTEAD OF trigger and check the text column(s) using IF UPDATE() and then joining the table to deleted on the primary key and sending the text value to the log table.  Then just do the update and other logging operations in the subsequent trigger code. 

If you use WRITETEXT to overwrite text values (which can be a good idea, along with the bulk-logged recovery model) this will not work, as the trigger will not be fired.