<mvt:item name="customfields" param="Write_Basket( 'my_made_up_code', 'This is my value I am going to write to the basket' )" />
<mvt:item name="customfields" param="Write_Basket( 'color', 'red' )" />
<mvt:item name="customfields" param="Read_Basket( '' )" />
<mvt:foreach iterator="basket" array="customfields">
&mvte:basket:code; - &mvte:basket:value;<br/>
</mvt:foreach>