Would "mein Schatz" be used between platonic male and female friends or is there a romantic connotation?
The common usage of "Mein Schatz" is reserved for people either in a romantic or in a family relationship (most common: mother -> child).
Though, you will also hear "Du bist ein Schatz" even when people are only in an acquaintanceship when someone did a very helpful action.
Example 1: You know a person from sports. You see them every week once, but do not have any further activities together. One day, you do a favor to them and the person is very thankful. Thus, they say: "Du bist ein Schatz".
Example 2: Someone is having some problems (whatever these problems are), and they know that you can give them some advice. They ask you, you listen to them and tell them what they can do. Because they are very thankful for your advice, they will finish the conversation by "Du bist ein Schatz".
The only condition is, of course, that they like you. If you're in the doghouse such an action would increase your reputation, but they will only say "Thank you".
Last but not least, sometimes very good friends will say "Mein Schatz" to each other, but that's meant in a slight ironic way. ;)
In Australia a family friend said Schatz to his wife for 50 years. I always took it to mean "sweetie", that you would only say to your 'love' or in modern terms BAE. So I did it with my fiancé.
But in answer to your question, it can be very ininimate but like any word it can be also used casually in very certain circumstances. E.g. shop attendants in England saying "ta love" to everyone.
A good English translation for "Schatz" is "treasure." So it refers to someone very "close" to you.
The first and most obvious use is for romantic partners. The second likely use is for family members, especially for parents to children, less so for siblings.
It's possible to use this for good friends (one of your top five) especially if one friend has been very helpful, but this is much less common than the other two.
"Schatzi" or "Schatz" is very risky if you don't know your acquaintances that well. Depending on context, it could be disrespectful, ironic or too suggestive. I think instances of people saying "Schatz" or "Schatzi" as a serious way of expressing endearment in friendship are very rare.
It is not advisable to say Schatzi to people you don't know very well because it can be used to indicate that you are not taking someone seriously.