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I found a way to integrate the search quite seamlessly in a quite easy way (took only about half an hour to do).
It's not very elegant, and I haven't tested that thoroughly, but so far is working great:
I tricked xcgallery to search the coppermine gallery instead of it's itself.
I installed coppermine in the same database as xoops, and I installed xcgal as well. Then I changed the search files in xcgall to search the coppermine gallery by hardcoding the table names in all the sql queries.
I also had to change /search.php so that the results are displayed pointing to coppermine instead of xcgal.
That's it! I even get the thumbnail to show up in search results!