There are many simple ball type valves to use for a fill line faucet. Think of your shutoff valves under your sink. Those are very small. Also, if your going to run a couple of lines, you could build a little recess in the wall with a trap door you can flip up to access the valve, and a small cutout where the hoses can pass through when it's closed. A small electrical access style box painted the color of your walls would work well. Ideally it would be concealed behind the tank/stand anyway. If you need to sell your house, you can remove it and patch over it.