Thought I might transmit the results of some fare prowling...
Croatia (OW service ex-DBV to LGW, DUB, MAN, ex-SPU to LGW, seasonal) is shown on on various fare tables as still being a cheap place to start an xONEx. So I emailed the Zagreb BA office to enquire, and got a response that they don't offer RTWs since no OW carrier flies to ZAG. I replied that I thought that DBV or SPU would qualify, and could they consult the rates dept. Their (very prompt) email reply was that there were no fares in Amadeus (which I expected).
Rather than ask again for them to consult the rates desk I phoned the AA RTW desk, who are usually quite good at quoting fares, and the CSR there said they're showing the following DONEx fares (sorry, I didn't ask about L or A):
DONE3 USD4400
DONE4 USD5000
DONE5 USD5900
DONE6 USD6500
These are the same as were being charged in WAW (and a dollar more than IST) until earlier this year, so if there's been an adjustment to the general OW pricing then AA doesn't know about it.
AA doesn't have any codeshare flights operating from DBV (no same-day connections to the US) so it looks like we'd be in the same situation as IST - deal with BA or nobody. So who knows if BA has re-priced the xONExs in Euros or something, and just not told OW.
Still, DBV is an attractive place, relatively easy to get to (even by ferry from Italy) and might be worth looking at for RTWs.
I do wish, though, that one could count on a fare being quoted by one OW member as being definitive, rather than dealing with noobs in sales offices or rates desks located on Pluto.