In analogy with the Web Terminal feature, a real view of the Zynthian GUI and a view of the Synth Engines GUI’s itself integrated on the webconfig seems to be in reach using VNC:
See also Can't use VNC
There is another way, using xpra, maybe this was done by the guy trying the integration of vcv rack
Yes, there seems to be a webclient for xpra as well
One caveat of xpra in combination with the Raspberry Pi is that there is a(buggy) limitation on getting a reasonable resolution on the client (I think 1024x768 is the max) which both in the case of vcv-rack and having multiple synth-ui’s with knobs all over the place might have it’s limitations.
I’ve tried to implement it, invested some time, but did’t work and I’ve left the idea.