The current Honda hybrids (Civic for sure and I think CRV also) work kind of that way - in slow speeds it works like a series hybrid and only in freeway speeds it connects the ICE to the wheels.

The claimed mileage is similar to Toyota's planetary power splitting thingie, no idea how it works in the real world.

You could also say that hydrogen fuel cell vehicles are series hybrids, since the fuel cell mostly charges a battery, which is much more flexible in what power it can provides and can be recuperated into, unlike a fuel cell which is one-way.