foreach(ListItem item in rbl.Items)
{
   item.Attributes.Add("custom", "custom");

The framework RadioButtonList implements the IRederInfoUser interface.
I found that creating a custom RadioButtonList which derives from the
framework one and also implements this interface can provider a
solution to this problem.

The only method that needs to be implemented is the RenderItem method.
This method is responsible for rendering the individual radio buttons
in the list. Example implementation below.

public class CustomRadioButtonList : RadioButtonList, IRepeatInfoUser
{
        void IRepeatInfoUser.RenderItem(ListItemType itemType, int
repeatIndex, RepeatInfo repeatInfo, HtmlTextWriter writer)
{
                RadioButton radioButton = new RadioButton();
                radioButton.Page = this.Page;
                radioButton.GroupName = this.UniqueID;
                radioButton.ID = this.ClientID + "_" + repeatIndex.ToString();
                radioButton.Text = this.Items[repeatIndex].Text;
                radioButton.Attributes["value"] = this.Items[repeatIndex].Value;
                radioButton.Checked = this.Items[repeatIndex].Selected;
                radioButton.TextAlign = this.TextAlign;
                radioButton.AutoPostBack = this.AutoPostBack;
                radioButton.TabIndex = this.TabIndex;
                radioButton.Enabled = this.Enabled;

                // Add custom attributes here.
                writer.AddAttribute("custom", "custom");

                radioButton.RenderControl(writer);
        }