Hi,
A category custom field would be a little bit more complicated to retrieve but you could be possible with some view modification to have such result.
The view "list_price" display the product price in several other views (like : product listing and product page)
By modify this view, you would be able to display another text when the price is "free". There is a dedicated part in the view which display the text "FREE_PRICE".
So, you can display the content of the custom field instead of displaying the "free" text.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.