Hey,
It's true that I am hacking away at Donncha's sitewide tags plugin.
I don't use the [gallery] tags but would guess that WPMu resolves the shortcode based on the current blog id? You asking the question indicates that resolving the shortcode on another blog (like the global blog) does not produce the desired results?
I have the same thing (I suspect) where the thumbnails of attached images are not displayed on the global blog (they are if I use a custom field "thumbnail", hence this thread to figure out how to pull postmeta...).
One surprising thing that did work, is setting the value of the the 'thumbnail' custom field (cf) key to:
http://sub4.blogdomain.org/files/2008/07/20080722_granny.jpg
"worked" just fine on the main blog! WPMu is smart enough to figure out the path to the original blog file repository...
http://sub4.blogdomain.org/wp-content/blog.dir/4/files/2008/07/20080722_granny.jpg
So, if you can figure out how it does that, you could insert the same process with your gallery tag... Easier said than done for us, I know!
I bet that if you look at what/where/when the postmeta data related to the gallery shortcode is generated, you'd be able to use one of the postmeta or custom field functions to add that data to the global blog like I added the cf 'thumbnail' stuff above.
BTW, when I delete an original post, postmeta I added is deleted as part of the core functionality so that's nice...