Well i didn't want to be too much complex in suggesting insert effects ... but i miss something, the system effects on Zynaddsubfx can be used for filtering other engines sound? or are you thinking about a downstream effector as a separate engine, always present at the end of chain but enabled or disabled on demand?.. (something like MOD UI but with audio plugins only...and maybe less cpu consuming)
Anyway i was thinking to system effects, 'cause till now (or i am wrong?) every engine, even being multichannel, must be loaded for each snapshot channel, included Zynaddsubfx.
Regarding Webconf, unfortunately i'm not very familiar with programming or something more complex than writing out a bunch of commands on terminal... but would like to try. How much is difficult?
[corrected] sorry, webconf is zynthian.local on web browser... never used