Statsig does indeed support experimenting with different elements such as email subject lines. You can create an experiment in Statsig and define multiple variants, each with a different subject line. This is essentially an A/B/n test where 'n' represents the number of different subject lines you want to test.
As for the use of multi-armed bandits or other selection algorithms to rotate through a pool of copy, Statsig does support multi-armed bandit tests. However, it's not explicitly stated in the documentation if this can be applied to rotating through a pool of copy.
You can use Experiments or Autotune in your email campaigns. Autotune is Statsig's implementation of the multi-armed bandit approach. For more information on Autotune, you can refer to the Autotune documentation.
For a practical example of how to use Experiments or Autotune in email campaigns, you can refer to this walkthrough guide.
For more specific guidance on your use case, it would be best to reach out to the Statsig team directly. They can provide more detailed information and help you set up your experiment in the most effective way.