It is true that Borax is water soluable. However, the borax has done its job by preventing the devitrification, so it does not matter whether it has or has not disolved, nor whether it is inside or outside.
Borax as a flux for paint in excessive quantities has the effect of corrosion on the paint or enamel it is mixed with. It is not actual corrosion, just that its effects are like that. The borax expands when wet. The expansion is very little, but over time "pops" off the paint - the time scale is 50-80 years. This happens on the inside of windows where the paint is. So it is not an inside/outside issue, just one of moisture.
But this irrelevant in kiln forming applications when attempting to prevent devitrification, or even to correct existing devitrification. The subsequent possible disappearance of the borax will not matter to the appearance of the piece. It has been reported that borax covered sushi dishes going through dishwasher cycles in a restaurant for years show no devitrification after the presumed disappearance of the borax. In fact, the proprietary devitrification solutions that contain lead would not be applicable in this food containing situation.
Other references to devitrification are:
Homemade devitrification solution
Description of devitrification