How to override the tree node template in ExtJS 5?

Question

I'm working around a bug in ExtJS 5.0, and to do so, I need to change the template used by the TreePanel.

Currently, nodes use normal anchors for all URLs, including Javascript calls:

<a href="javascript:doSomething()" role="link" class="blah">label</a>

For JavaScript URIs, I need to use onclick instead to avoid the bug:

<a onclick="doSomething()" role="link" class="blah">label</a>

I'm trying the following override and it appears to have no impact on the HTML:

Ext.override("Ext.tree.Column", {
    cellTpl: [ "Hello World" ]
});

What am I missing ? Shouldn't this override cellTpl?