@bobmccrory There is now a draft PR in Pluto to have this more advanced communication between JS and Julia.
It is still a work in progress but your mandelbrot example is much easier to implement with this additional functionality.
I used the julia code from @lungben in Mandelbrot with Plots.jl for computing the colors in the plot.
You can check the notebook code in my comment to the PR itself, but you do need to use the Pluto version of the PR branch to start the server if you want to test this yourself.