It is being done inside of the cljs.js namespace.
I've been looking into an experiment that might lead to a solution https://github.com/planck-repl/planck/commit/1be2eab8bce95a0f1439ee5317fa47ff45176487
Ooh, cool! :)
Hmmm... I've tried to understand what's going on the with the eval/`load-string` namespace thing but it's beyond my understanding. When I look at the code, it looks to me like planck.core/eval delegates to cljs.core/eval which itself delegates to cljs.core/*eval* with the intention that *eval* will be bound at runtime. The thing is that I can't see anywhere where this runtime binding happens.