Seems to me like a case of that (for ye olde Warhammer) typical phenomenon of 'Look, we need this fantasy race in our world, too!', followed by the later 'Oh well, nobody cares about these. Drop them!'.
Never heard about any Mordheim-related info (far too late, I think; by the time GW did Mordheim, nobody was still bothering with those earlier-edition relics), but the old rulebook for WFRP1 has a page in the bestiary I have included at the end of this post for your convenience.
The German Lexicanum also has a page about them (the English one does not). If you do not speak German, no need for despair or language courses; the text is nearly an exact and word-for-word translation of the WFRP page.
But the Lexicanum page does mention Warpstone Magazine issue 17 as a source, so I think that might be something to investigate further (although considering the very underwhelming amount of information the Lexicanum offers here [and the very unofficial nature of Warpstone Mag itself], that source might be not too much, either).
Moral: Not much, and I think they never were more than a curiosity, but I do like digging through old stuff like this.
The page from the WFRP1 bestiary (bear in mind the statlines are not so compatible with the ones we are used to, and it is more an approximation than a real conversion, but for modern Warhammer, they might be a tad faster than Dwarves [/ Dwarfs
], with a higher Initiative, but also not as tough and have a lower WS and Ld - which in the end puts them about on par with a regular Human or even a strong Halfling who changed his bow for a sword. I think the differentiation here should come more with the special rules rather than the statline):