Great news! This was a major concern for me.
On the 26th of this month AWS announced: https://aws.amazon.com/about-aws/whats-new/2019/11/shared-vpc-now-supports-network-load-balancer/
I actually missed this announcement amongst the fervour of all the others and was looking into sharing subnets today. There are still constraints left. One notable one really being the Amazon FSx 🙁 Not as much of a show stopper as the NLB was for me however.
For more on the unsupported services for VPCs that are shared refer to the docs here:
Thanks for fixing this on up AWS. The Network hub will be that much more viable in my reference arch from now on.