As a commenter missed spoof-source support from 2.0, I got my act together for an afternoon hacking session and implemented it in the 2.0.x tree.
The packet generation for IPv4 packets was straightforward, I could simply "forward-port" it from 1.6.x, after creating the proper place for it in the 2.0.x tree. IPv6 support was a bit more difficult as libnet has a nasty bug with IPv6 and UDP. I was looking at my code for hours, when I tried Google codesearch to check whether I was doing something wrong. Then I've found this. Note the comment above the libnet_build_ipv6() call. I also disabled UDP checksumming and now it works like a charm. The last libnet release was almost 3 years ago, it is not reassuring that its webpage is also down. Too bad, libnet is a fine piece of software, but now as its website is down where can I point syslog-ng users to download libnet from?
By the way, I did not announce syslog-ng 2.0.2 here, it was released about a week ago. Nothing fancy, primarily portability fixes and a more important fix in the usertty() destination. Grab it from the usual place.