From a wood working perspective.....yes, but only to be trusted with a sealant or gasket. Adjoining materials being flush do not always account for an "air-tight" seal. It is always recommended that you use a sealant or gasket when building any type of structure that is made to hold water or be air-tight. Besides, when working with computers all it takes is one drop of water to ruin the whole project. Granted mineral oil is a different monster in comparison to water, and plexi is a different monster in comparison to wood...but I would just assume play it safe when it comes to any types of liquids.
If someone has done the dovetail alone to seal liquid I would love to see how you went about it.