Averaging over one pedal revolution is sufficient -- and if your sensor is mechanical, its own inertia could do the averaging on it's own -- no software needed.

Still, I'm not aware of anybody actually selling such a device meant for use on a bike. They certainly do sell devices meant for finding the slope of something, and it's easy enough to do with a level and some rulers, but I've never seen anything meant for a bike. It could be done, however -- it mounts to your top tube (close to level already) then is calibrated somehow (ride in circles and it measures the angle constantly if it's electronic, or manually with a level) ... but I haven't seen it.

My 705's grade measurement is certainly adequate, but it could be done a lot more simply and cheaply. A spirit level would do it, once mounted on a bar level (with shims if needed) and with marks put on the glass to indicate the grade when the bubble is at various spots. Ideally, it would be long enough to give readings for angles between -20% and +20% or so accurate to within 1 or 2%.