(Reliefs, plural, in your context.)

"Xilo-collage-relieve" is Berni's own term, and he coined it to draw attention to a very conscious personal technique that he himself developed. I therefore feel that a similar term should be used in English, rather than a descriptive translation.

As Lorena points out, Christie's lot description actually uses the term "xilo-collage-relief", and they are not alone; so does the Museum of Fine Arts, Houston, for example:

"Beyond its parodic portrayal of the cross-section of Argentinean mid 20th century society, the Ramona series stands out for its introduction of an innovative printing method that the artist called xilo-collage-relief. This method is based on traditional woodcut relief, yet it expands the medium to incorporate materials from everyday life (fabrics, metal ornaments, coins, buttons) that produce a "collage" effect on the actual impression."
http://prv.mfah.org/collection.asp?par1=12&par2=&par3=75&par...

The term contains three elements, the first of which, "xilo", obviously refers to "xilografía". "Xylography" (with a y) is a perfectly correct term in English, but it is rarely used; it is nearly always called woodcut printing. ("Woodblock" also exists, but this term is used for the traditional Chinese and East Asian use of the technique; the Western equivalent, which can also be called xylography, is conventionally known as "woodcut".)
http://en.wikipedia.org/wiki/Woodblock_printing
http://en.wikipedia.org/wiki/Woodcut

As noted in the MFAH text quoted above, Berni's technique is based on woodcut relief, which is a well-established term, but incorporating a special kind of collage. Its particularities are explained in the rest of the sentence, so there is no need to incorporate that explanation in the translation of the term (and in fact it is inappropriate to do so).

I think there are two options. One is to do as Christie's and MFAH have done and Anglicise the artist's own term, but if so, it should be "xylo", not "xilo". My preference, however, would be to use "woodcut" instead of "xylo", and call it "woodcut-collage-relief" (with hyphens, as Berni has it). So it's a kind of hybrid of woodcut collage and collage relief, both of which are well-established techniques:

On collage relief, see pp. 49 ff here:
http://books.google.es/books?id=a7eRj_wbwzkC&pg=PA52&lpg=PA5...
John Ross, Complete Printmaker

More refs to collage relief:
https://www.google.es/search?num=100&hl=es&tbo=d&q="collage ...

Plenty of examples of "woodcut collage" too:
https://www.google.es/search?num=100&hl=es&tbo=d&q="woodcut ...

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Note added at 1 day19 hrs (2013-01-23 14:42:40 GMT) Post-grading
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It's a pleasure!