there are several ways how to implement this in instant jchem. Java plugin which you suggest might be the most elegant, but also the most complicated way. Another option is to use calculated fields. This can be used as an interface for some simple groovy code. Yet another possibility is to call external program from the groovy script. This may be the easiest and least error-prone possibility.
That said, I encourage you also to assess whether you really need to use Lee and Richards algorithm. If this is for validation or reproducibility reasons, then go for it. In other cases, I am less sure. If you simply went for speed, you could go along with one of ChemAxon's current algorithms. If you were after accuracy, then you should probably go for some simulation tool.