There is a move away from the use of the apostrophe in the 'adjectival' (rather than the 'true possessive') sense you mention here. However, the tendency is to just drop the apostrophe rather than switch to an unusual-sounding singular modifying noun:
Working Mens Club
Mens Clothing Department.
( at http://dict.leo.org/forum/viewGeneraldi ... de&lang=de :
Style Manual for Authors, Editors and Printers (John Wiley & Sons):
Nonpossessive and generic phrases. In phrases such as drivers licence, travellers cheques and visitors book, the plural noun is descriptive rather than possessive. As it describes an association with the following word rather than any direct ownership, no apostrophe is necessary. Some other examples:
After primary school, she went on to the girls grammar school.
The various proofreaders marks are shown in an appendix.
Phrases such as drivers licence and travellers cheques have become merely generic ways of referring to common items. If used in a [non-, EA] generic sense, however, an apostrophe is still needed:
The young driver's licence was cancelled.)
Sorry - that link seems to have expired. However, a parallel recommendation can be found at: http://grammar.about.com/od/ab/g/apostrophepunctuationterm.htm
That's gone too. Try this extract from The Associated Press Guide To Punctuation
By Rene J. Cappon (and thanks to sumelic):
Descriptive Phrases Without Apostrophes
Don't use apostrophes in such primarily descriptive phrases as a New
York Mets outfielder, a teachers college, a writers manual, a
childrens book, the agencies request. As the AP Stylebook helpfully
notes, the apostrophe is usually skipped if 'for' or 'by' would go
better than 'of' in a longer version: college for teachers, manual for
writers, request by the agencies.
In descriptive names, some organizations or institutions use the
apostrophe while others don't. For instance, Diners Club , and
National Governors Association. [both updated; EA] [The Long Island
Writers' Guild, Redbud Writers Guild, The Harlem Writers Guild -
examples still valid, EA.] Consult your house style." (Rene J. Cappon,
The Associated Press Guide to Punctuation. Basic Books, 2003)
So, the awkward-looking spellings seem to be triumphing over the awkward-sounding alternatives.