AFAIK "ajouré" and "à résilles" are one and the same thing, until your client comes up with an explanation of any difference.

From my notes:
Résille Grid system (false ceiling support); open-cell / eggcrate ceiling Dicobat
Plafond ajouré Open cell ceiling [Barbour91,322], eggcrate ceiling

If you feel the need to provide alternative terms in English too, then "cell or eggcrate ceiling".

egg-crate ceiling - A cell ceiliing

cell ceiling, egg-crate ceiling - A suspended ceiling of closely spaced vertical blades forming a square grid, which hides a/c and lighting systems while acting as a diffuser for both ...
[The Usual Source]

résille ... 4/ Un plafond-résille est un faux-plafond formant paralume, composé d'un réseau quadrillé de lames.
GB : egg-crate ceiling, cell ceiling
[The Usual French Source]





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Note added at 4 hrs (2007-10-21 12:57:42 GMT)
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SECOND THOUGHTS
When I encountered "plafond ajouré", it was, TTBOMK, a cell ceiling.
Thinking about it though, there are other kinds that might be described as "ajouré", such as the very common suspended strip ceilings (strips of aluminium with a narrow gap between) or the rarer perforated ceiling (see http://perforatingdesigns.com/exterior/5.html).

This as opposed to false ceilings made of mineral-fibre panels which are close-jointed (though some tiles will have small perforations).

"open-work suspended ceiling" gets far too few ghits to be an accepted term, but might be a safe bet nonetheless.