Custom field type editable in Excel mode?

I have written a custom field type with this definition (to be used in SP2010):

<FieldTypes>
  <FieldType>
    <Field Name="TypeName">MyField</Field>
    <Field Name="ParentType">Number</Field>
    <Field Name="TypeDisplayName">My Field</Field>
    <Field Name="TypeShortDescription">My field description</Field>
    <Field Name="AllowBaseTypeRendering">TRUE</Field>
    <Field Name="UserCreatable">FALSE</Field>
    <Field Name="FieldTypeClass">$SharePoint.Type.3e27f5e8-348d-40c6-afcc-87306b2bb3ac.AssemblyQualifiedName$</Field>
    <Field Name="Sortable">TRUE</Field>
    <Field Name="Filterable">TRUE</Field>
  </FieldType>
</FieldTypes>

Basically, this custom field extend the native SPFieldNumber to provide a custom UI:

[Guid("3E27F5E8-348D-40C6-AFCC-87306B2BB3AC")]
public class MyField: SPFieldNumber
{
    public MyField(SPFieldCollection fields, string fieldName)
        : base(fields, fieldName)
    {
    }

    public MyField(SPFieldCollection fields, string typeName, string displayName)
        : base(fields, typeName, displayName)
    {
    }

    public override Microsoft.SharePoint.WebControls.BaseFieldControl FieldRenderingControl
    {
        get
        {
            return new MyFieldControl
            {
                FieldName = this.InternalName,
            };
        }
    }
}

The custom field works fine in the standard newform.aspx and editform.aspx. However, when I create a datasheet view, the field is read only:

What have I to do to make the field editable in the datasheet view?