I don't use KDE, but I'd like artsd to start at boot. Arts has some software channel mixing that the equivalent alsa stuff can't do, you see. Anyway, it seems that the good solution would be to rc-update add arts default, or something similar, to achieve this. No such rc script exists.
Not gonna happen unless you fork Gentoo and put KDE into boot runlevel yourself... :=)
Not counting that arts is not suited for this use, it requires to be run as root as it uses some IPC method to communicate with apps. If you really want, you should be able to use esd in this way.
Quick change of platform in bugzilla. Sorry about the email.