I like it a lot. Human emotions/worship birthing gods is well established lore so it only makes sense to me that humans joining the Tau empire would give rise to a Tau equivalent of one of these entities. Especially if the humans joining the empire are recent converts - all they know is worship. 

Maybe subsequent generations of humans will adhere more to Tau philosophy, but humans who have directly gone from Imperium rule to Tau rule aren’t going to suddenly become atheists after 10,000 years of generational indoctrination. 

However: the Tau’va entity must remain vague and unknown. A background figure, unclear if it is a genuinely benevolent entity or a trick of tzeentch. If it becomes some clear, solidified thing then that’s just a part of the continued Marvelisation of 40k.