If it goes good I can see Parson not expecting it because the enemy has done something unexpected
How is this counting as unexpected, for Parson? Jetstone has a hatomancer and a dollamancer. I don't think that the Dollamancer making some shiny new accessory can be counted as "unexpected" at all. (Unless Parson doesn't know what Dollamancers can do. It is a bit unexpected for us because we weren't thinking about what else the Dollamancer can do besides golems, but I don't think it's something big enough to throw a wrench into Parson's plans - it seems like it's just a powerup for Tramennis. Probably an anti-air or anti-uncroaked powerup given the fight, but still.)
Either way how will Trem deal with his casters doing this behind his back?
I'm not sure that Trem will have to 'deal with' all that much. What's going to happen? The casters spend a bit of juice to make him a new toy or weapon. It's not earth-shattering in terms of strategy and impact. It's not like Tramennis is going to have to deal with suddenly being down two casters - both have plenty of juice, they haven't done anything this turn yet. I think he'll deal with it the same way he dealt with Ford's elaborate plans - routinely.
I'm interested in what'll come out of the casters spending their juices... I'd guess that either the expenditure of the juice will be worth while and save the day; or the lack-of-juice will cost them the battle. Either/or.
I'd guess the other way around. It's a text update. I think the most likely thing is that the accessory will add some flavor to the coming fight, but that this won't have a significant tactical impact one way or the other. Sort of like Ansom's and Ford's tactical planning, or Wrigley's zeal.