I think "(un)power" is just fine.

It certainly is possible to speak of "economic power." Here are some references to academic papers that use the term in their titles, from Google Scholar:
http://scholar.google.com/scholar?q="economic power"

"Marketing power" is also used in an academic context:
http://scholar.google.com/scholar?q="marketing power"

Now, "unpower" is an English word, although it's archaic:
http://www.thefreedictionary.com/unpower
That definition is from Webster's 1913 dictionary.

Here are a couple of occurrences in Middle English:
http://quod.lib.umich.edu/c/cme?didno=AJH1649.0001.001;rgn=f...
http://quod.lib.umich.edu/c/cme?didno=AFB3713.0001.001;rgn=f...

For a scholarly text, I'm not sure that the fact that it's archaic matters that much, especially since it seems that the term will be used in the context of a play on words with "power" in the body of the text, and it is quite acceptable to use "power" as a noun in the title of an academic paper on marketing and economics. Adding the quotation marks is a way of suggesting the wordplay to come.

So I'd go with the client on this one.

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Note added at 18 hrs (2012-12-21 18:35:21 GMT)
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Another thought.

"Lack of power" and "powerlessness" are descriptive. It is possible that the author of the paper is thinking less of the negation of a particular state or place as described "objectively" but rather of negation in an active or even dialectical sense.

For example, the idea could be that, with respect to a given system, what appears as "power" actually includes its contrary as a position (rather than as a state or condition, if that makes any sense) or as the grounds of its subversion.

(One, but far from the only, illustration of this sort of thing is found in dialectical materialism, where:
"...contradiction, as derived by Karl Marx from Hegelianism, usually refers to an opposition inherently existing within one realm, one unified force or object. This contradiction, as opposed to metaphysical thinking, is not an objectively impossible thing, because these contradicting forces exist in objective reality, not cancelling each other out, but actually defining each other's existence."
From: https://en.wikipedia.org/wiki/Contradiction#Dialectical_mate... )

To say again, the author may want to put an explicit focus on an "A vs. ~A" type of relation.

I obviously have no idea what the paper is actually about, of course. I'm just trying to imagine why the author might want to have a wordplay based on a prefix of negation in the title.

It really depends on what the paper is actually about!

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Note added at 1 day8 hrs (2012-12-22 09:13:47 GMT)
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Thank you very much for the additional explanation in the discussion section.

I continue to think that (un)power is a fine option, with the parentheses to highlight the negation. In other words, "(un)power" rather than "unpower." I think the parentheses will let the reader know that it is a question of negation rather than an unusual use of an obsolete word.

Adding "un" to a word to define something by the negative has been used in a other registers. For instance, there was an ad campaign for the lemon-lime soda 7UP with the slogan "7UP, the Uncola" (and note that "Uncola" is used as a noun):
http://imprint.printmag.com/wp-content/uploads/Can2.jpg

And from http://www.baerpm.com/blog/?p=221 :
"In the late 1960s and early 1970s 7-Up teamed with J. Walter Thompson to create the “Uncola” campaign. The purpose was to mimic the rebellion that characterized much of popular culture and society, specifically targeting the growing youth market."

and

"What does it mean to be uncola? Let’s break it down:

Un-: Not
Cola: a carbonated soft drink colored usually with caramel and flavored usually with extracts from kola nuts

So uncola is not a carbonated soft drink colored with caramel and flavored with extracts from kola nuts, seems like a pretty solid positioning strategy."

Also see:
http://www.7up.com/page/history/
http://philadelist.com/2010/02/uncola-and-why-i-find-adverti...

Further, the "Uncola" campaign included different puns involving "un":
https://secure.flickr.com/photos/wafflewhiffer/2782653605/si...
https://secure.flickr.com/photos/47607517@N04/8133016516/siz...

The campaign was revived in the 1990's, as well: http://www.commando.com/ARCHIVE/Post10.html

Now, I don't mean to suggest that "(un)power" would somehow imply the specific messages of the "Uncola" ads (no more than I think it explicitly implies the Middle English references). It's just another kind of precedent. It's my attempt to explain why I don't think "(un)power" would be completely confusing for readers who are already used to reading about "economic power" and "marketing power."

All of this is just one opinion. There certainly are other ways to approach the question.