How to flatten a specific group of form fields on a page?

You can flatten specific fields but you can't toggle back and forth like you do with your readonly script because once the fields are flattened they no longer exist. The flatten function flattens all annotations (including form fields) on a page or a range of pages. This function has an optional input parameter to exclude annotations that are non-printable so in order to flatten specific fields you would have to write a custom script that pushes the visibility of all annotations into an array, sets all the fields you want to exclude to non-printable, flattens all pages, then returns the remaining fields to their original visibility. Or you can download a tool that does all of this for you from www.pdfscripting.com.

Supposing the PDF form has no comments or annotations, just form fields, you can use a script such as the following to implement the procedure described by David:

var list = new Array(); 
list = [ //fields to be flattened
    "personalInfo",
    "notes",
];

var fields = new Array;
for (var i = 0; i < this.numFields; i++) {
    fields[i] = [
        this.getNthFieldName(i),
        this.getField(this.getNthFieldName(i)).display
    ];
    this.getField(fields[i][0]).display = display.noPrint;
}

for (var i = 0; i < list.length; i++) {
    this.getField(list[i]).display = display.visible;
}

this.flattenPages({
    nStart: 0,
    nEnd: this.numPages - 1,
    nNonPrint: 1,
});

for (var i = 0; i < this.numFields; i++) {
    for (var j = 0; j < fields.length; j++) {
        if (fields[j][0] == this.getNthFieldName(i)) {
            this.getField(this.getNthFieldName(i)).display = fields[j][1];
        }
    }
}