I can't deny the word "diorama" exists in English (just looked it up in the dictionary), but I personally have never come across it before and I doubt it would mean anything to most people.

I'm assuming the figure is standing up, of course! Without seeing the "diorama" you're talking about I can't be absolutely sure, but since you say it is a "poza" I think this might work.

Here's an example of one:

http://cwilliamshk.trustpass.alibaba.com/product/11163295/St...

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Note added at 34 mins (2006-04-10 10:52:02 GMT)
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According to this site, "diorama" in English means:

"...a partially three-dimensional model of a landscape typically showing historical events, nature scenes, cityscapes, etc. for purposes of education or entertainment."

http://en.wikipedia.org/wiki/Diorama

It seems to be used more in English to mean a complete scene including a group of people or animals (e.g. a nature scene) with a background, but I can't find anything to suggest it would be used for a figure/model of a single person.