curly,

You can do this with the current iactionable inteface

where you would normally put the edit url string put in a string to open the modal popup that you want to show

so for example i have a function on my page to show a modal popup created by the modal popup extender

function modalAuditDisplay(newDateValue,newAuditValue)
        {
            var hiddenDATEfield = $get('<%=hfDate.clientID %>');
            var hiddenAUDITfield = $get('<%=hfAudit.clientID %>');
            if (hiddenDATEfield && hiddenAUDITfield)
                {
                    hiddenDATEfield.value = newDateValue;
                    hiddenAUDITfield.value = newAuditValue;
                    __doPostBack('<%=hfDate.clientID %>');
                    $find("ModalBox").show();
                }
        }

in the iactionable item i would put something like

Get
                Dim Actions As New Entities.Modules.Actions.ModuleActionCollection
                Actions.Add(GetNextActionID, "Add War", Entities.Modules.Actions.ModuleActionType.AddContent, "", "", " modalAuditDisplay('12/1/2007','12/1/2007')", False, Security.SecurityAccessLevel.Edit, True, False)

                Return Actions
            End Get

HTH