Can I override the default categories for platform controls in XAML as well ?

It can be done with the following piece of code.

[XAML]

<TextBlock x:Uid='TextBlock_2'>
<TextBlock.ToolTip>
<TextBlock
x:Uid='TextBlock_3'
Localization.Attributes=
'$Content(ToolTip Readable Unmodifiable)'>
Microsoft Corporation
</TextBlock>
</TextBlock.ToolTip>
Windows Vista
</TextBlock>