I'm working on the 2.5 release where there'll be a new metadata param to set a alwaysOnTop property for a specific container; I can't make it public jet but here is the changes you should do to the mbContainerPlus.js code for this feature:

line 96:

if (container.metadata().alwaysOnTop) container.css("z-index",100000).addClass("alwaysOnTop");

line 530:

replace the function jQuery.fn.mb_BringToFront with:

  jQuery.fn.mb_BringToFront= jQuery.fn.mb_bringToFront= function(){
    var zi=10;
    $('*').not(".alwaysOnTop").each(function() {
      if($(this).css("position")=="absolute" || $(this).css("position")=="fixed"){
        var cur = parseInt($(this).css('zIndex'));
        zi = cur > zi ? parseInt($(this).css('zIndex')) : zi;
      }
    });
    $(this).not(".alwaysOnTop").css('zIndex',zi+=1);
    return zi;
  };

On your container just add:

{alwaysOnTop:true, buttons:...}