I tried on Sunday at Brixton, where the clerk seemed to know about it
but refused to do it since there was a long queue. The fact that I
had stood in the queue for 20 minutes cut no ice and he told me to go
away and come back another time.
At Oval the clerk had no idea what I was talking about; I showed him
the page of the fares leaflet and after some consideration he said
that the capbility had been built into the system, but they couldn't
put it on yet (seemed a reasonable response, since he had clearly not
been trained in how to do it.)

Finally went to Green Park on Monday evening, where there was no queue
at the ticket office, and though the ticket clerk hadn't heard of it,
after my showing him the fares leaflet he had a go on the system and
managed it without too much difficulty. Still took long enough that
there were some annoyed people in the queue behind me though!

It seems a great deal - means that if you don't travel in the AM peak,
daily capping will be slightly less (IIRC) than buying a weekly Z12
travelcard. With the proviso of course that it's valid on almost no
NR services south of the river. The recent extensions to oyster
validity seem to have made the system even more biased towards North
London - as the map at http://www.tfl.gov.uk/assets/downloa...G-08-01-02.pdf
shows.