I can't comment on production viability today but if you assume that language itself is irrelevant then it becomes clear that runtime and engine level is the way to go.

We spend quite a lot of time conceptualizing around safe self mod and to build apps that can change at runtime. We ended up using custom Lua VM, type system to catch mistakes, declarative homogenous infrastructure and actor model (erlang inspired).

Actor model provides not just a good isolation but also it's much easier for AI to reason (since most of components are not that large), we already able to use it to write quite complex systems with ease.

Another upside - in actor model you don't really need any of this fluff with cron jobs, queues and etc, all the logic naturally maps to indended architecture, making implementation of agents _very_ easy.

https://wippy.ai/en/tutorials/micro-agi It takes 4-5 files to create mini sandboxed AI agent at top of actor model with ability to modify own toolkit while having system guardails and no access to core filesystem.