Replacing value axis value labels with arbitrary text

While it's not possible to directly establish value overrides with some text, it can be achieved using Guides.

First of all you need to disable regular value axis value labels:

valueAxis.labelsEnabled = false;

Then you just need to add Guide objects for each value you need.

// GUIDE for 100
var guide = new AmCharts.Guide();
guide.value = 100;
guide.lineColor = "#CC0000";
guide.label = "hello";
guide.inside = false; // this puts labels outside plot area
guide.lineAlpha = 0; // this makes the actual guide line not to show
valueAxis.addGuide(guide);

Here's a full working demo.