I was looking for some information myself just yesterday and came across this page from Apple: About HTML widget creation in iBooks Author - Apple Support
But it's probably not going to make it too easy for you, so don't get your hopes up too quickly! 😕
A number of third party suppliers provide widgets. Some free, some commercial, some custom made. Given that FileMaker has been around for a few years now, you might find someone already has something you can use. (I think the key will probably be whether or not you have your FileMaker database accessible via the web, rather than using the native client to interact directly with it.)
I found my solution by using Tumult's Hype 3, which I already had (but hadn't used for ages). It has a specific Export to HTML 5 > iBooks Author Widget option. I have no idea if it can do what you're after, but there's a free trial you can try before committing to a purchase and support forums where you might be able to find like-minded developers who can point you in the right direction.
http://tumult.com/hype/download/
Good luck!