Besides for reasons of tznius, is there any other “issur” involved in urinating in front of another person (or where a person would be able to see you.)
The concern is in fact one of tznius, modest behavior.
It should be noted that tznius in this regard is not just a “midat chassidut” but rather the letter of the law, quoted in Shulchan Aruch O:C 3:2. This is based on the Gemara Brachos 62a which speaks about modesty in the washroom with great severity. In fact the opinion of the Sema”k, Tnius is in fact one of the 613 mitzvos of the Torah, see Biur Halacha ibid. s.v. Yehey.