I'm not sure if it could harm the ducks or you. What if you ran your grey water into the soil before the reed bed allowing it soak in and let the watershed bled it into the reed bed instead of running it through?
Failure isn't failure if a lesson from it's learned.
I don't know much about reed beds, but last year I had ducks on my pond and, as the pond got smaller and their poop accumulated, I started to notice them getting wet feather and getting sick. I ended up having to fence them out of the pond until the fall rains came and did a "water change" on the pond. They had access to fresh clean water, in both pails and trays, but they spent so much time in the pond that they still were getting sick. They got better once I fenced them out of the pond.
I would definitely keep an eye on them, especially looking for wet feather.
My take on ducks and reed beds is that the reed bed system is an effluent treatment system, and as such it is best fenced from livestock, children, visitors etc. including ducks. Grey water systems are much less of a contamination risk, but better to err on the side of caution. Once the effluent has passed through your grey water reed bed, then by all means route it to a pond where the ducks can swim and enjoy it. The grey water irrigation will help to keep the pond topped up. Ideally if you are doing this you may wish to put in a biosand filter set-up at the outlet from the reed bed or the pond inlet to screen out any residual bacteria (although given the abundance of duck droppings that may be obair in aisce (Irish for wasted work)). This is similar to Johathan's suggestion, only I'd typically make the grey water filter about 25m2 for a 3-bedroom house if you've got dish washers, washing machines etc. and all the usual detergents going in. If your usage is less you could well get away with a smaller system.
Robert's point has merit if you have high bedrock or impermeable clay subsoil which will stop the water migrating down into the groundwater. Otherwise the percolation area will let your grey water leave vertically and not provide you with irrigation water for your pond or habitat wetland area.
Interesting to hear Nicole's experiences. Good to have a regular throughput of water, so the larger your reed bed the cleaner it will be prior to the pond (within reason. If it's too large, then in dry weather the evapotranspiration rate will outstrip the water and rainfall inputs and you'll loose your water into the air).