Can I add javascript to a plugin config page so I can alter backend xml file?

[WebStuff]

Versions: PHP 5.4, VM2.0.18, J2.5

Hi,
I would like to add extra fields to a shipment plugin config page "on the fly" as it were.
In VM1 this was easy as the backend config was loaded from a php/.cfg.php file so I could just add an "Add" button and a "Delete" button for extra parameters. Then use javascript to update the plugins cfg.php files from the backend.
On VM2 the config for plugins is loaded from an xml file, so is there a place that I can add fields or a function that hooks into the backend onload event so I can throw some js powered buttons into the backend plugin config screen?

Any help greatly appreciated.

[WebStuff]

Or if someone knows of a plugin that already does this then I could take a look at the code.
Please post a link if you do know of one.

Thanks

[WebStuff]

I am trying to get this for a non-commercial UK postcode shipping plugin if that is any incentive to reply.