I have also contacted AWS support, who acknowledged the lack of support for this, and confirmed that an internal feature request already exists for this.
You could argue, that since there is no API for this, that this isn't a real gap. But I'll keep it because the functionality is there for other runtimes
The AWS::Lambda::Function.Code.ZipFile property is not supported when the Runtime is "nodejs10.x" https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-lambda-function.html https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-lambda-function-code.html#cfn-lambda-function-code-zipfile
Forum post discussing the issue: https://forums.aws.amazon.com/thread.jspa?threadID=303166&tstart=0
I have also contacted AWS support, who acknowledged the lack of support for this, and confirmed that an internal feature request already exists for this.