In a developer sense, this functionality is already baked into the plugin. The actual generation of passwords is left to the wp_generate_password() function, which is one of the WordPress pluggable functions (core functions you can override easily by including functions with that name in a plugin or theme. Copy the original wp_generate_function() source from wp-includes/pluggable.php into your theme's functions.php, then adjust it to meet your password standards.
As an added benefit, this is the function that WordPress core calls whenever it needs to generate a new password, so updating it in this one place will ensure that generated passwords, be they from core or WP Password Generator, will conform to your password policy.
By the way, great call on using 1Password - I'm in that app every day between my laptop and phone. Finally coughed up for the family license when 1Password 4 came out so I could get my wife and brother using it.