Well FG, you did get some response but.......
Never the less, I can't recall a member doing this to the stock hull. I have seen it done when the hull was rebuilt but obviously it's much easier. I know on my CCP, there was foam on the outside of the stringers so running a pipe there would be tough if your Osprey is the same.
Cutting a hole in the tank coffin isn't terrible. Seal around it to reduce the chance of water intrusion. I have a much smaller well on my deck. It originally was mounted on the platform bracket of my CCP and had a 2" drain. When I moved it over to the WAC, I used the existing live well drain which was much smaller(I'll check tonight). It has worked fine. The key is pump flow. Original pump (on the CCP) was a Rule 450 GPH. The current pump is a Shurflo Bait Master which flows about 4 GPM so you see it isn't a huge amount. Rated for a 35 gal well, it would be small for you but it may be the answer. Lowering the flow will allow you to reduce the drain diameter and you could then fit the hose below deck.
If you have the well, can you try the overflow on the deck? Place it on the deck with a short section of the smaller hose and fill it with a garden hose. Experiment with the garden hose flow to see what happens. After you find the combination of flow between the garden hose and drain that allows the well to function, I'd suggest then measuring the flow with a 5 gal. bucket and a watch. You can then get a fair idea of the max flow that the reduced drain will handle and size the pump accordingly. Upside, you haven't cut holes in the deck to find out if it's doable.
This is what I did with mine before I cut. I used the existing drain so I was limited (like yourself) to a smaller hose diameter(I had to also reduce the well overflow drain fitting to allow for the smaller hose).
I drain the well when done by connecting a hose and running it on the deck to the splash well drain. Works fine.
Lastly, the idea works well in theory and in practice for me because I'm not trying to keep huge quantities of fragile bait alive so if that's the reason for the large well, you may be best served with JD's option.
Good luck.
