I was wrong in how I described these fields, they are actually set as type="text" not hidden, but the other details are the same.
Here is an example of what one of these fields looks like in the form:
<input name="addblock" id="addblock" type="text" style="display:none" value="" />
You would then want to add in a regular expression type of validation, check the box for allow blank then use this regex:
In the User Registration Solution Pack there are two of these text fields each with their own validation.