What I ended up with was doing this, which works fine.

public class SaveComposer : IUserComposer
{
    public void Compose(Composition composition)
    {
        // Append our component to the collection of Components
        // It will be the last one to be run
        composition.Components().Append<SaveComponent>();
    }
}

public class SaveComponent : IComponent
{
    // initialize: runs once when Umbraco starts
    public void Initialize()
    {
        ContentService.Saving += ContentService_Saving;
    }

    // terminate: runs once when Umbraco stops
    public void Terminate()
    { }

    private void ContentService_Saving(IContentService sender, ContentSavingEventArgs e)
    {
        foreach (var content in e.SavedEntities)
        {
            // Delete versions older than 48 hours
            var date = DateTime.Now.AddDays(-2);

            sender.DeleteVersions(content.Id, date);
        }
    }
}