Yes, this can be implemented.

Could you please share a XAML snippet demonstrating how you intend to use this property? This will help us better understand your scenario and requirements.

Hi sure,
In my usage, it is to be able to simply center Icon or other element I am injecting while using rich text tag (by using RichText::SetTryCreateInlineCallback).
If I create an InlineUIContainer to put inside an image or an icon, it currently align it's baseline to the rest of the text, which is correct behavior but WPF allows us to do this (I wouldn't write it like this in xaml but I would be creating it in cpp to create something to that :

  <TextBlock Foreground="White" Margin="10" FontSize="12">
Center alignment 
<InlineUIContainer BaselineAlignment="Center" >
    <Rectangle Height="30" Width="30" Fill="Yellow"/>
</InlineUIContainer>
  </TextBlock>

And allow us to not have to care about FontSize as it will always be vertically centered (if not I will have to manually calculate negative margin to try to align it myself. Other Alignments are also nice to have but not as essential (like superscript for injecting tooltip "infobubble" for example)