Ronald,
You could use JTidy in a java action see http://jtidy.sourceforge.net/index.html
With a smaal java action you can get the escaped values as in the sample code below:
String test = "<p>action & prices</p>";
InputStream stream = new ByteArrayInputStream(test.getBytes(StandardCharsets.UTF_8));
Tidy tidy = new Tidy();
tidy.setXHTML(true);
tidy.setPrintBodyOnly(true);
tidy.setShowErrors(0);
tidy.setQuiet(true);
tidy.setShowWarnings(false);
tidy.parse(stream, System.out);
This code will take in the string with html and return the corrected version to the System.out, with some effort this should be easliy integrated into your Mendix app as a java action call.