The strength required for a tub is high - water is pretty heavy stuff. Blowouts are not nice...
Your biggest problem will be finding a closeable drain that will work with your liner...be prepared for sticker shock as they're few and far between and therefore dear.
Call the people at
www.schluter.com. They have (or did) a special order drain that would work with their Kerdi membrane. I don't think they show it on their website...basically, they're hand made (and very nice) to order, but you'll probably have to wait for it. There are limitations on the shape that you can have and still get the membrane to work well - compound curves don't work particulary well.
Another possibility is
www.wedi.de . They do do custom stuff and may have a drain.
You may not be able to hook up an overflow, so check with your local inspector if that would be allowed.