Thank you Mike for the answer although it wasn't what I hoped for. I eventually figured out myself whay the posted code wouldn't work (my UserControl is not a Grid and hence it cannot be designed with the GridDesigner and as far as UITypeEditors go I wouldn't know how to display the editor since I cannot see any public method in it that will do that).

In any case I spent almost two days on this and I managed to scramble something that might pass as a solution:

using System.ComponentModel;
using System.ComponentModel.Design;
using System.Windows.Forms;
using System.Windows.Forms.Design;
 
namespace Jpmc.PB.Stable.ApplicationTechnicalServices.UIServices.ComponentsLib
{
    [Designer("Jpmc.PB.Stable.ApplicationTechnicalServices.UIServices.ComponentsLib.MyControlDesigner, Jpmc.PB.Stable.ApplicationTechnicalServices.UIServices.ComponentsLib")]
    public partial class StableGridX : UserControl
    {
        public StableGridX()
        {
            InitializeComponent();
 
            _saveFilterBtn.Click += (sender, e) => _theGrid.SaveCurrentFilter();
            _resetFilterBtn.Click += (sender, e) => _theGrid.Rows.ColumnFilters.ClearAllFilters();
 
            _namedFiltersCtl.DataSource = _theGrid.NamedFilters;
            _namedFiltersCtl.DisplayMember = "Name";
 
            _namedFiltersCtl.SelectionChangeCommitted += (sender, e) => _theGrid.ApplyFilter((NamedFiltersEntry)_namedFiltersCtl.SelectedItem);
        }
 
        [Browsable(true)]
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
        public StableGrid TheGrid
        {
            get { return _theGrid; }
        }
 
        public static implicit operator StableGrid(StableGridX ctl)
        {
            return ctl.TheGrid;
        }
    }
 
    public class MyControlDesigner : ParentControlDesigner
    {
        private StableGridX _stableGridX;
        private IDesignerHost _designerHost;
        private bool _disableDrawGrid;
 
        public override void Initialize(IComponent component)
        {
            base.Initialize(component);
 
            _stableGridX = component as StableGridX;
            
            if (_stableGridX != null)
                EnableDesignMode(_stableGridX.TheGrid, "TheGrid");
 
            _designerHost = (IDesignerHost)component.Site.GetService(typeof(IDesignerHost));
        }
 
        public override int NumberOfInternalControlDesigners()
        {
            return 1;
        }
 
        public override ControlDesigner InternalControlDesigner(int internalControlIndex)
        {
            return _designerHost.GetDesigner(_stableGridX.TheGrid) as ControlDesigner;
        }
 
        protected override bool AllowControlLasso
        {
            get
            {
                return false;
            }
        }
 
        protected override void OnPaintAdornments(PaintEventArgs pe)
        {
            try
            {
                _disableDrawGrid = true;
                base.OnPaintAdornments(pe);
            }
            finally
            {
                _disableDrawGrid = false;
            }
        }
 
        public override bool CanParent(Control control)
        {
            return false;
        }
 
        protected override bool DrawGrid
        {
            get
            {
                if (_disableDrawGrid)
                    return false;
                
                return base.DrawGrid;
            }
        }
    }
}

Now this designer allows me to invoke the grid designer on the portion of the control that represents the grid and it correctly generates the code in the .designer.cs file.

Cheers, hope this helps others!