Following up on some confusing comments in the DD-WRT forums, see http://www.dd-wrt.com/phpBB2/viewtopic.php?t=167815 (with a screenshot of settings)
In the model-specific thread I learnt that my VPN issues are unlikely to be model-specific after all, so I could have dared to post it here in the first place. http://www.dd-wrt.com/phpBB2/viewtopic.php?p=736784
Sticking to the wiki for my Linksys E900, I currently have eko's 19342 big. Are the VPN servers supposed to work there? If not, is there a newer version where they are?
By the way, the Mac OS VPN client complains about communication problems, if this helps anyone. (So it is not an authentication issue, e.g.. Though I was worried that I somehow misunderstand Krikkit's comment from 2006: "The admin password of the router is inserted into chap-secrets by default!")