Not sure if this is covered by some larger feature (e.g. Javadoc code
completion), but perhaps the following would not be too hard to implement, and
would be very useful for anyone editing Javadoc:

If a @see or @link tag is present in a Javadoc comment, try to look up the
target. (Paying attention to imported classes and so on - Retouche APIs should
be quite valuable here. And make sure to handle '#' correctly - i.e. '.' only
separates package components from classes, and I think classes from nested
classes, but '#' must be used for fields, methods, and constructors.) If the
target is not found, add a warning to the document.