Pa Zong and Sarana were backer-funded characters, designed to have specific routes so as to keep the budget in terms of writing and CGs for them, under control. The same goes for all the other backer maids in the game - their "screen time" was limited, because they were designed to be in the game only as supporting characters or simply brief cameos.
As of right now, there are no plans for DLC for the game, as we technically didn't even hit Miyuki's goal from the Kickstarter after one large dropped pledge and a couple others brought it down. The total shown there is not the total raised as it was more like 13.6k when all was said and done (as stated in my update on December 28th 2016).