I believe is not done. The test case there converts String to Integer, to represent it as Lob, but these are both basic regular types. If instead of a String you use something else (some custom Class), it doesn't seem to work.
For example, here I use a class called Imagem, and convert it to a String:
The above code works OK as long as the field is not a @Lob (which means the converter itself is correct). But as soon as I add @Lob to the field it stops working. For example:
Then, when I try to save, it should see a String (the converter gets called), but it still sees an Imagem, and then I get this:
If I save it to the database without @Lob, and then add the @Lob and try to open it, I get this: