Here's a quick search box for your OPAC. Keyword only I'm afraid, but it at least shows that searching your catalogue from elsewhere is possible:
Search Blackburn with Darwen Libraries
It's gone a bit skew-whiff, but it works. It was a couple of minutes' work, based on looking at how DS Galaxy builds queries and how they can be reconstructed using a web form. What it doesn't do is keep your branding etc. in place because Galaxy uses frames for that sort of thing and I don't know how to do that.