Custom field types in xml for plugins

[Phil]

Hi, really stuck on this one and would really like some advice.

I am using a custom field type and following the examples in the docs
http://docs.joomla.org/Creating_a_custom_form_field_type
I made the folder and placed the custom field type in it and then can
see this in the xml of my plugin, I am using a custom field type as I
want to run php in the params so I can bring back a server value.

That aside I used the city example in the docs
http://docs.joomla.org/Creating_a_custom_form_field_type
This all works fine until I save the plugin, the value selected is not
added to the params in 2.5 database table.
If I use normal JFormField types then when I save the plugin then the
values are added to the params column in the _extensions table.

I appreciate that the idea is to use the custom field types in our
components etc but can we not use them fully to get and store values
in xml fiels that define plugin parameters, if not what would be the
best way to execute a php lookup that will show in the plugin
parameters?

Many thanks for reading this far