Can an application that requires JRE 6 run in Rosetta 2?
Azul is already released openJDK Arm64 8 binaries including JRE. Java8 is backward compatible so can run all previous Java versions.
I believe it's possible to setup environment so that java app will use it instead of Apple Java. HomeBrew suggests:

For the system Java wrappers to find this JDK, symlink it with
sudo ln -sfn /usr/local/opt/openjdk/libexec/openjdk.jdk /Library/Java/JavaVirtualMachines/openjdk.jdk

or with macports just
sudo port install zulu-jdk8 and /usr/bin/java finds it automatically