Crosswinds are never demonstrated on contaminated runways. It is complex and too risky to do full-scale tests. Furthermore certification rules such as FAR 25, EASA CS 25 don’t require this. The information you will get from a manufacturer is advisory material only (if you get anything at all!). Each manufacturer has its own way of deriving this information. Normally they will use engineering simulation models to come up with advisory crosswinds on contaminated runways. These models often can’t accurately model gusts so everything is for steady winds only. Most of the models have not been fully validated on contaminated runways and rely heavily on model tests done by NASA, RAE and other organisations.
See also pages 32 and 57:
http://www.nlr-atsi.com/eCache/ATS/14/919.pdf