Masternodes could have an additional config flag which would allow donations to be sent to the darkcoin foundation.
Something like this:
With 10 being 10% to a hard coded Darkcoin Foundation address. So if your MN received a payment of 1 darkcoin you would receive 0.90 dark instead.
Edit: It would also have to be defaulted to 0. You can not force people to donate.
Im totally in for this idea - happy to donate 50% as I said previously - now is not the time to be greedy. Donate now and watch it pay you back 10 fold later!