How to modify on the fly in a snippet

In some pages I use a snippet to dynamically charge some information.
In case A, this snippet display a list of items
In case B, it display only one item.

I want - in case B only - to purge all other content of [[*content]] to only display the item.
In other word: replace all [[*content]] with the snippet result, but preserving and render all other elements of the template.

Using $modx->resource->_output allow to replace all output and doesn't help me.

Has anybody done this before? Without using a plugin on event like OnWebPagePreRender...

Thanks for any help