update: this is what i found out so far.
since i was unclear about which 28mm lens would match my experience with the 50mm summilux v.2, i looked at hundreds of pictures on flickr, taken with the different 28 lenses.
i found most pics not to be tagged precisely, so checking flickr was less informative than i had hoped. still i was able to draw some conclusions. i try to describe my very personal impression - this highly subjective and only depending on my "taste of the day".
some care needs to be taken when looking at pictures in flickr. first, it's obvious that the image quality suffers from small format and compression artifacts. then, pictures uploaded to flickr usually are selected and postprocessed by the respective owner - what you see is not the "raw" product of the camera/lens, but an edited and refined "essence".
all conclusions drawn must be seen with these considerations in mind.
it was fairly easy to make out differences between several groups of lenses. this does not mean, that each and every picture clearly showed the signature of the respective lens. honestly, for most of the pictures i saw, i would be unable to identify the lens used to take it. but still, there was a general trend regarding the look that i am after.
rating:
-- not my thing at all
- i don't like it
+ i like it
++ bingo, that's it
the "state-of-the-art" group:
- summicron: while capable of stunning results, for each picture i liked, there were at least two pics i disliked strongly, generally far too 'clean'. (phew, me and my bank account are happy to find that i do not like this lens!)
-- elmarit asph: no surprise, like the summicron, only more so. extreme dislike.
- ultron f/1.9: less apparent than with summicron, but still not to my taste
the "modern" group:
- konica hexanon: similar to elmarit v4, too 'clean'
- elmarit v4: mostly unremarkable, high contrast.
+ zm biogon: while modern in looks, still more appealing than the others.
- cv 3.5: inhomogenous. B&W acceptable, colour somehow not.
the "classical look" group:
+ ultron f/2: a bit "rough", but lively and appealing
++ elmarit v3: the nicest of the whole bunch
of the lenses of the final group ("antiques"), there were too few samples to come to a final verdict:
canon ltm: a bit soft
minolta: left no special impression
summaron: very pleasant drawing!
the surprizes:
the summicron. i had expected a more pleasant rendering
the summaron. simply wow.
the f/1.9 ultron. in my memory, it was nicer than it appeared to me now
the result - i'll need to find a v3 elmarit and try it out. usually i don't like tabbed focusing...