I've recommended it for years!
If you search this forum for "sea water intake odor," you'll find a bunch of discussions recommending teeing the head intake into the head sink drain line. In fact, some sailboat builders plumb their boats that way to eliminate a hole in the boat and to save the cost of a thru-hull and seacock.
So by all means go for it! And btw...I just posted to that thread. But you might want to hop back in with a link to this one:
http://forums.sbo.sailboatowners.com/showthread.php?t=123472
However, I'm not sure I recommend using hard pvc for vent line though...'Cuz that's a hard one to support to prevent shock and flex. Sanitation hose is just fine for vent lines. I suggest that the vent thru-hull be replaced with an open bulkhead thru-hull that can be backflushed every time the boat is washed and/or the tank is pumped out to prevent it from ever becoming blocked. You might want to search this forum for "holding tank vent line maintenance."
