I would like to add my own fields/extend existing fields. Instead of messing with the default fields folder (wp-content/plugins/piklist/parts/fields), may I suggest that you add support an extra folder to the theme / plugin folders. eg:
myplugin/parts
/meta-boxes
/widgets
/settings
/fields
If a file (eg select.php) is put in the fields folder above, it will be loaded instead of the default one.