On reading your admirable guide to tax returns I think I may have spotted an important omission:

On page 6, Charges et Imputations Diverses (which this year is to be headed Charges Deductibles - I think) there is a line actually entitled: Deductions Diverses, DD where you can deduct cotisations sociales made directly, where these are not deducted from salaries etc.

This includes payments made into the CMU (CGI, art.156-II, 4°, 10° et 11°).

The people concerned should receive from the URSSAF, before declarations are due, an Attestation de Versement showing how much they have paid during the tax year, which they should send with their declaration.
PO

For 2007 income, the box DE is for the deduction of part of the three social charges that are paid in the previous tax year, usually mainly on income from bank interest, dividends, and rental income.

Box DD is intended for alimony payments (where the arrangements were set up prior to 2 November 1959), and for other claims generally to do with historical buildings.

If you missed the April edition, the helpsheet is available from the site by clicking here, priced €5.