In the interest of giving credit where credit is due, my avatar is a scan of a hand-drawn caricature by cartoonist Jim Naylor which was done at my company's 20th anniversary dinner.
This is covered in the Traders and Barbarians 5-6 Player Extension rulebook in the section for this scenario.
T & B 5-6 player rules (p4) wrote:
If, after set-up, you build any settlements or upgrade any settlements to cities during your turn, 1 camel is placed after you finish your turn and before the special building phase begins. If, during the special building phase, one or more players build or upgrade any settlements to cities, another camel is placed after the special building phase is finished. If a camel is placed after the special building phase, the player who just finished his turn also starts the voting round. This player may also place the camel if the established voting procedures do not lead to a majority of votes for one player or several players combined.
There are two oases now, as starting points for a total of 6 caravans. When you place a new camel, you are free to decide from which oasis to start your caravan and which caravan you want to extend with the new camel.
So, in summary, your option 3 - one camel before special build, one after, with the player whose turn it is getting to choose on split votes in either case.