As far as i know, kava is a GABA-ergic substance. And GABA-ergic's generally seem to dull the effects of psychedelic's.
If you're looking for a sedative that goes well with psychedelic's, i think cannabis could be it. Cannabis hugely amplifies the effects of basically every psychedelic. Especially shrooms. So large quantities of cannabis should maybe be avoided, but small quantities go very well with shrooms. It also counteracts some of the side-effects, like nausea.
For some reason, the stoning effects of cannabis tend also to get amplified by shrooms.