I can't reproduce it.
I built the class you attached into a dll (release build, with VS 2005), and placed it under "[LLBLGen Pro v2.6 installation folder]\TypeConverters" folder.
And opened an old Northwind lgp project file into the LLBLGen Pro v.2.6 Designer, and whenever I click on a string field in any entity, I see the new GuidTypeConverter in the "Use different .Net type" combo box.