Inserting custom code into post

I’m running a gallery on my WP install which is triggered when an ul with a certain ID apperars on the page. Since it would be awkward for the client to write HTML himself to trigger the gallery can I make it easier for them? The gallery would not appear on every single page, so it cannot be fully automatic, let’s say the code appears for some category only, or completely manually via click of a button?

Any ideas?

Cheers!