Hi Kristof,
I checked ORM Designer2 configurations for Doctrine2 and currently there is no support for unsigned at all ;-). So if you have "unsigned" in your ORM property tree, it's probably your extension or it's imported from your project.
Anyway, to support unsigned in correct way you referred, I have extended configuration file Doctrine2.ormd2.cfg.xml at line 74 in this way:
<struct name="Field">
....
<struct name="options">
<attribute name="version" type="string"/>
<attribute name="comment" type="string"/>
<attribute name="unsigned" type="bool"/>
</struct>
</struct>
After this change you wil get following ORM Properties:

When you export your model, you will get:
/**
* @ORM\Column(type="integer", nullable=true, options={"unsigned":true})
*/
private $age;
This update will be available in the next release. In the meanwhile you can edit your configuration file manually and add this one definition to line 74 by yourself:
<attribute name="comment" type="string"/>