From the website of the French Academy:
Use of accents on capital letters
It can only be deplored that the use of accents on capital letters is inconstant. A certain trend tending to the ommission of accents is observed in manuscript texts. Some often suppress, in typography, all accents on capital letters under the pretext of modernism, but in fact to reduce the costs of composition.
It is appropriate to observe, however, that, in French, the accent has full orthographic value [sic]. Its absence slows down reading, provokes hesitation in pronunciation, and can even lead to error. The same applies to the trema and to the cédille.
In good typography, care should be taken for the systematic use of accented capital letters, including the preposition À, as all good dictionaries clearly do, beginning with the Dictionary of the French Academy, or the grammars, as Le Bon Usage, by Grevisse, and also the National Printing Press [l’Imprimerie nationale], the Bibliothèque de la Pléiade, etc. As for manuscript or typed texts, it is evident that their authors, out of concern for clarity and correction, should have all interest in equally following this rule.
So, then: Ecône or Écône?