Turns out it was actually working on the published form.

I used this code to achieve it (rudimentary but can be fancied significantly:

public override List<PreValue> GetPreValues(Field field)
    {
        List<PreValue> _lResult = new List<PreValue>();

        try
        {
            string[] strings = HttpContext.Current.Request.QueryString["edates"].Split(',');
            int sort = 0;
            foreach (string value in strings)
            {
                PreValue _pvPreVal = new PreValue();
                _pvPreVal.Id = sort;
                _pvPreVal.Value = value;
                _pvPreVal.SortOrder = sort;
                _lResult.Add(_pvPreVal);
                sort++;

            }
        }
        catch (Exception ex)
        {
            Umbraco.Forms.Data.LogHelper.Error(
                "Get values fromrequest provider: " + ex.ToString());
        }

        return _lResult;
    }

In the backend, this doesn't generate actual pre values, not even if I add things to the URL.

However, in the live form, this works as expected and a list of checkboxes will get populated correctly.

Maybe this is of use to some people...