Are spouses allowed to touch one another in public (when the wife is not a niddah) or is this a violation of tzniut? I am asking specifically regarding touch that is not in any way sexual or even affectionate.
According to RaMBaM one should speak as little as possible to ones wife in public under the bounds of modest behavior, and from what I have learned, corresponding with my Mori's as well as students of Rav QafiH, it is not far off to render if speaking is to be avoided so, too, is touching.
"The guiding rule is that he should speak only words of wisdom or in connection with acts of kindness and the like. He should not speak to a woman in the marketplace, even if she be his wife, or his sister, or his daughter." Hilkhoth De'oth 5:13 (7 in the Vilna)
This should be obvious but this also indicates a conduct non-sexual in nature, due to the examples given of one's sister and daughter. But as far an actual statement in halakha that says one may not on the grounds of ssniuth, I know of none. Seems it would be a stringency fitting of a talmid hhakham.