You can't have it as a child object and editable on a history ticket without going through the edit history feature of tickets. When the parent MBO is read-only the Maximo framework automatically cascades down to the child objects and won't let the record be saved. It'd be sort of an ugly hack but as long as you don't depend on the parent object being the ticket, you can create a dialog where the main object is your custom object with the fields editable. Then you could add an action for a user to open the dialog.
------------------------------
Steven Shull
Naviam
------------------------------