I'm not suggesting that the conditional logic live in `RuntimeServices` - the code to inject your default executor/engine into overrides (if necessary) would live here, as a replacement for the `_scriptExecutorType` and `_scriptEngineType` fields.

As far as I can tell, this is the only place that you pull a value out of `_overrides` other than uses of `RegisterOverrideOrDefault`. Feels odd, but maybe it's just me.