When using a tag with a relative url, e.g.
<base href="app/" />
The use of any <a href='#'> throughout the site causes the href on the anchor tag to be rewritten according to the relative path, which then becomes clickable to an invalid url.
I can replace <a href='#'> with <a href='javascript:void(0)'> and then the browser ignores rewriting the target url according to the prefix specified in the base tag - and it works as expected (doing nothing and causing no navigation.
But I was wondering if there was a more general way to cause the '#' url target to be excluded?
No. A base element with href attribute affects all URL-valued attributes, by definition. Your page probably needs a redesign. The base element as such is seldom a good idea, and a elements that are not meant to be links are even worse.