I'm aware, that it isn't an issue of unicodedata, it was just the way I 
came to try such a "modern" unicode character.
I also see, that tk works pretty well in finding an appropriate font 
(e.g. compared to wx, which I use more often) - it took me quite some 
time to find another clearly unavailable character - PHAGS-PA 
LETTER ... :-), which however behaves like I'd expect - a square is 
displayed.

Regarding capital eszet/sharp s missing in windows' charmap, this isn't 
anything special, as its character database seems to be quite ancient 
(acording to http://www.babelstone.co.uk/Software/BabelMap.html it 
might be Unicode 2.0, instead of the current 5.1). Capital sharp s was 
added in this last version of the Unicode standard - 5.1. (The 
mentioned tool also has a tool to find a font containing a given 
unicode block.)
In the unicode database, there is a "crossreference" to the small sharp 
s, but I'm not sure, how this info should be interpretted, it's a 
different item than the plain lower/capital pair.

However, is there something to look into to check, what might be 
misconfigured on my system?