sharing designs without sharing source files

I don't believe so. People have used .vqms and partitions for this type of thing, which is easier since you don't have to set up a license for the end customer(and not as well protected, because of that). But both of those methods provide post-synthesized netlists, so you will lose all parametrization and force a specific synthesis(i.e. the end user couldn't re-target a differnet family).

The only thing I can think of that might be worth googling is "obfuscation". Basically this is an algorithm that re-names all of your nets, registers, parameters, etc. to completely unintelligible names. So someone could still look at the RTL, but it's much, much more difficult to read. I don't know if there are any free tools out there for this, but I believe Altera used to do it and I've heard of other companies doing it, so wouldn't be surprised if something like that existed.