Actually if you get the fluid down inside the stack, or down inside the hole into the smoke whistle, there is really no way it can get on the boards. (If you miss the hole it will of course run all down the side of the loc.) At the entrance to the smoke unit there is a small brass "funnel" that fits around a corresponding boss in the boiler casting, and this prevents fluid from overflowing or running anywhere other than into the bowl. If you overfill the fluid bowl, the first place the excess will run is into the fan chamber. From there it will certainly overflow onto whatever is below it in the immediate vicinity. If you invert an overfilled engine so as to remove the shell, it will definitely run out of the smoke unit and will make a bit of a mess inside the boiler. But this is generally pretty easy to mop up once you have the boiler off.
As far as making a removable panel so as to visualize the amount of fluid, this may not be very practical. If the fluid bowl was made of clear plastic you might have a chance, but the wadding would prevent any real level indication I would guess. Remember, there is not supposed to be free liquid in the bowl; it is supposed to be completely absorbed by the wadding. Plus the bowl gets pretty hot in operation due to the resistor heat, so plastic may not be a good bet. Model aircraft guys often use clear plastic fuel tanks, just so they can see the fuel level, but they are not dealing with wadding, or heat.
While we are on the subject, an engine whose smoke unit has been used, even if only once, should NEVER be stored inverted or on its side. Residual smoke fluid will most definitely run out and make a mess over time if it is stored in any position other than right side up. I suspect that most of the mess on the boards in the OP's Bigboy likely occurred during shipping (with the fluid bowls full, but NOT necessarily over-filled). During shipping it would have been in god knows what positions, quite possibly never right side up.
Just my nickle's worth.
Rod