My suggestion was to add a new role *capability* to WordPress. The capability could be called something like 'edit_cimy_field'.
Then, in your plugin, you could change the setting 'Can be modified only by admin' that is in the admin pages for your plugin to read something like 'Show the field only if the role has the 'edit_cimy_field' capability'.
Then in the code for the plugin, rather than checking if the user is in the 'Admin' role, you could check whether the user is in a role that has the 'edit_cimy_field' capability.
And therefore, your plugin wouldn't be tied to specific role *names*, but would cater to the "extensible role model" I think WordPress is moving to. But as you say, to make modifications to which role has the 'edit_cimy_field' capability, people would need another plugin for managing roles. But if you were to make it so that the 'Admin' role gets the 'edit_cimy_field' capability when your plugin is installed, people wouldn't necessarily need a role management plugin.
The MailPress plugin does exactly this - it creates a few of it's own WordPress capabilities. And it also has a 'role manager' feature too. It'd be worth taking a look at the plugin to see how hard this option would be. Option #1 above may be simpler though. Or course it's up to you though.