Got it.

Is there a way to instruct glow (without frontend code changes), that if the model parser encountered a layer it cannot parse, ask the backend if it can parse it, instead of failing immediately?

Depends by what you mean without frontend code changes. The current master cannot do this. We would need to make changes to do something like, if the node isn’t supported in Glow IR, then check the backend if it supports it as an ExternalFunctionCall or something similar, and if so load it as such. I.e. this would require a single code change to the frontend so that such layers could be loaded as something like an ExternalFunctionCallNode. It would then be possible to load them and then the backend would be responsible for its execution.