Hi chesapeakebaybe...

As Badflash already mentioned...I also, don't know if you'd have enough nutes in the water. If you tried a foliar spray, you may end up adding too many nutes in the return water and create an algae bloom, in your part of the lake.
There would also be the issue of dissolved oxygen, more of a concern if you did a raft system. Perhaps you could run the water through a de-gassing tower before you have it go to the plants.
I guess keeping the intake on the pump clean, wouldn't be much more difficult then one in a typical closed AP system.
You may want to check the regs in your state/county....there may be laws in regards to using lake water (?)
Good luck...