It can be made to work, I just tested this in an edt in 4.2.1 and it works (chrome browser), but not if the tooltip
is a child of the outputText, or if the tooltip is outside the edt.
<h:outputText id="xxx" value="hello"/>
<rich:tooltip mode="client" showDelay="850" target="xxx">
The author of this blog warns against the use of tooltip inside a datatable without going into details about why:
if the performance problems relate to the header as well as table body. I generally try to stick to title= in tables.