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.
I would even offer this option to my MN Clients - would be very cool
Even just hard code it into the code
and this is looking at the next bump in MN percentages