Mark, your idea is sound. What you describe sounds very much like my locker is set up. There is a huge space under it - far too large to fill with chain or whatever, so a reinforced fibreglass tray sits there about level with the V-berth with a stout locker door set into the for'd bulkhead to allow access. The drain is in the lowest point, and as the floor of the tray has been shaped so it is a bit higher to port, and sloping down slightly to starboard, the drain ends up then coming out on the starboard side of the bow about 2 feet above water level with a rear-facing deflector over it. It works well. How do I have all this so fresh in my mind? Let me tell you what happened as a typical weekend boat project - for me anyway.
I had decided to end for end the chain - as one does, right? It was new 9 yrs ago, and the galv is not looking its best now in the half that I use a lot, and looked quite a lot newer and nicer at the unused end. So far so good. Wrong...! First mistake was removing the anchor. Why, because then I found the swivel bolts connecting the chain to the anchor were frozen, and the stupid hexagonal allan key type undoing thing just stripped the hole in no time. Ok - no probs - took it off by cutting last chain link with angle-grinder I had at least remembered to take - anticipating probable trouble. In any case, most of you don't believe in swivels, so what the heck.
Then, when I went to do what should have been a breeze, that is feeding out the chain, swapping it end for end, then feeding back again, but I found the winch wouldn't work - in spite of all efforts to address possible corroded contacts etc from sitting for longer than usual unused when we went overseas, it refused to go. This is a winch which has never failed to work first flick of the switch the whole time I have owned her, so not expected....!
Then I thought, ok, no problem, a bit harder and longer to do, but can still do it by just man-handling the chain out on deck and reversing it. Wrong again. After 9 yrs the (new & galvanised) shackle I used to attach it to the inside mount in the locker was so frozen it was going to be a hacksaw job - in a very confined space, naturally, as with all things boating...so a real bast***d of a job to get free. (I attached that when I bought her, back in the days before I learnt about the nifty trick from this forum of attaching the bitter end of the chain with a length of rope rode long enough to clear the winch, to facilitate a quick cut and departure in an emergency and to stop the winch being damaged by coming up against rigid resistance in a long feed out.)
About this time I also discovered that near the bitter end - that is the part I could not now free easily, not only was that shackle a bit corroded and unwilling to undo, but one or two of the links not far down the track were also a bit dodgy-looking from sitting in a pool of salt water long enough for some rust to develop. Darn...! As a chain is only as strong as its weakest link, it made no sense to swap it around and put those dodgy links down near the anchor did it? So about there I decided I had done enough damage for one day. I was back where I started, with everything best left exactly as it had been, but with a winch not working, (something else to sort out and no doubt expensive), and the anchor cut away - unnecessarily it now became clear, although easily re-attached, sans swivel of course... ho humm..... another day enjoying this boating thing....is this making you feel any better walt...?
However, coming back to Mark's problem. I did take the opportunity, while all the chain was out on deck, to clean out the bit of mud and rubble in the locker, and wash it out with fresh water, and it all flowed out the drain beautifully - so there...something went right...a small but important point I think...?