Hi Ottie, you are welcome.
I took a look at this but I can't figure it out...
You have first to enable permalinks in WordPress (using for example %postname%) then on NextGen Gallery (wp-admin/admin.php?page=nggallery-options), checking "Activate permalinks" options. You can also change the gallery slug name changing the "Gallery slug name" input field.
That creates something like:
http://www.yoursitename.com/yourpagename/gallery-slug-name/tags/yourtag
but this doesn't works very well... seems that
$nggRewrite->get_permalink (a NextGen Gallery function) under my nggpowertags_GetGalleryTags function returns all the tags with the same slug and the filtering doesn't works at all, even if I put the right slug in the URL.
So I think it's a probably bug into NextGen Gallery and, I'm sorry but, I can't debug it.
Hope you take it as is and if you like to help me on development, I'll really be happy :)
Cheers and happy new year!