Based on other cases, I guess the "unicode-style" solution would be to add a new codepoint DUTCH CHARACTER IJ OR ÿ which is cased and sorted accordingly.

Of course then you'd have to teach people to use that character when writing instead of just typing "ij". And one day, there will be someone who, for stylistic reasons, needs tight control over when it's rendered as "ij" and when as "ÿ"...