Is Tsang widely used as a romanization for the surname Zang in Mainland China?

No, it is not. Tsang is the Cantonese romanization of the surname 曾, whose Pinyin version is Zeng. When Zang is seen as a surname in mainland China, it refers to another Chinese character, which is 臧, whose Wade-Giles version is Tsang. So, you could see that this surname 臧 is written as Tsang in Taiwan. Here are several examples to illustrate what I mentioned.

Here is a picture of Margie Tsang, or 曾华倩 in Chinese, an actress from Hong Kong.

Here is a picture of Kenneth Tsang, or 曾江 in Chinese, an actor from Hong Kong.

Here is a picture of Zeng Li, or 曾黎 in Chinese, an actress from mainland China.

Here is a picture of Zeng Mengxue, or 曾梦雪 in Chinese, an actress from mainland China.

Here is a picture of Zang Xiaoyu, or 臧晓玉 in Chinese, an actress from mainland China.

Here is a picture of Zang Cong, or 臧聪 in Chinese, a singer from mainland China.

Here is a picture of Tsang Jui-Hsuan, or 臧芮轩 in Chinese, an actress from Taiwan.