I don't think you can place the callout without the leader.

Just recently on a project, I needed an "L" shaped callout but, as you know, callouts can only be square/rectangular. So I used a filter to change the linework of the callouts to be white, and used detail lines to make my "L" shaped bubble.

By making the lines white, it also made the leader white; however, the detail reference stayed visible. You still need to use detail lines to re-create the callout outline. Maybe this method can help you?

I will play with the filter to see what you are talking about. But it sounds like it will be even more effort than what we currently do.

When we have a text note in a detail and we want to reference another detail we will use a detail callout bubble. This draws the eye instantly to it and also has the advantage of being "live" (so if the detail is moved on sheet the detail number changes parametrically). So, for this purpose the callout leader is unnecessary. I understand that in Revit 2013 there is a new feature that allows for a "live" text callout (like detail such and such on sheet so and so). But for earlier Revit products we aren't so lucky.

I suppose the only way to change this would be to alter the program code itself somehow.

I will play with the filters. Thanks for the suggestion. :-)