I agree it would be wise to have a consistent method for romanizing Chinese names, which apparently is decided to be Hanyu Pinyin. But whether it should be applied to all names which are "supposed" to be Chinese? I don't agree. We're already inconsistent on the matter thanks to all of the Rot3K permutations. I'm pretty sure everyone who knows the source material will agree that all of their names are "supposed" to be Chinese. But we don't actually use them in that fashion, and admittedly part of this is due to convenience to avoid extremely cumbersome qualifiers. Let's look at some examples from this thread.
I'm assuming (since I haven't seen the source show), that tianzi is used as a tag because in the actual show, it is actually pronounced in that fashion? As in, they speak Chinese rather than Japanese? So this is an example where we do use the Chinese romanization because that's the only one that makes sense. A more familiar example to me is Hong Meiling, which also fits the "Her name is actually Chinese and rendered as such".
Syaoran/Shaoran/Xiaolang, on the other hand, was converted into Japanese. As much as Xiaolang is accurate to his name were it in Chinese, the fact is he was given a Japanese name, not a Chinese one. He might be from Hong Kong, but his name is not.