Why does the ramrod have to go in the gun? I suppose you could put it along side the barrel, instead of under it, but you would still have to cut away some wood on the foregrip to allow it to be the correct length.
Instead, how about simply carrying the ramrod into the field like I do with my shotgun? I run it down my back, under the strap for my possible's bag, and the end into the top of my jeans, in the small of my back. The other end sticks up above my head some, and has a doorknob type handle up there. I just reach behind my neck, and grab the rod, and pull up on it to remove it. The jag does bump the strap on my possible's bag a little, but the strap is wide enough that I don't get the jag hung up on it before it lets it pass. When I am done reloading my shotgun, I stick the rod back down my back, and into my jeans, Pick up the gun, and go on. My double Barrel is a little lighter to shoot without the ramrod adding its weight to the front of the gun, and that seems to be okay for me.
Just a thought. Underhammers do pose problems. You can drill a hole through the stock and then carve out( rout) a space in the forestock for a ferrel that is covered by the wood. The forestock screw could then be screwed into a thickened portion of the ferrel , and the ferrel can be welded, or soldered to the barrel as normal. I would use a steel ring only about 3/8" wide for that stock ferrel, and it would be thicker than the ones used under the barrel. That way, I could tap and thread and screwhole in the outer rim of the ferrel for mounting the stock and stock " bolt ". The stock is merely hung from the barrel, and does not provide any pressure or uplift to the barrel, as a modern rifle stock would do.
The ramrod would then go through the ferrels under the barrel and through the stock, stopping at the end of the barrel, and just before the nipple, leaving only a couple of inches sticking out in front of the muzzle. The forestock would cover the one ferrel, but it would serve as both a ramrod guide, and the mount for the forestock screw.