Is it possible to use IPv4 prefixes (subnets) as "public IPs" in the network interface of an AWS EC2 instance?

1. Allocate Elastic IP address using your BYOIP Pool
2. Create a new Elastic network interface
3. Associate the newly allocated elastic IP with this Network interface
4. Attach the Network interface to an EC2 instance

You can create and configure network interfaces and attach them to instances in the same Availability Zone The number of Network Interfaces you can attach to an EC2 instance depends on the instance type. Here you can find a table https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html#AvailableIpPerENI

You cant assign public IP/Elastic IP directly to EC2.

For your use case, what you would need to do is as follows.

Simply delegate your desired Private range for example 172.20.30.0/28 to an EC2 first, as per the link you shared.

After that create 1-to-1 mapping of your BYOIP to each IP in 172.20.30.0/28 range. This step will be done in associate Step of Elastic IP where you can select private IP. You will have to do it for each ip individually. With that your EC2 will be able to use BYOIP Pool.