Just like how when something is uncalled for, it becomes an uncalled-for something when used attributively, you need to use a hyphen to make a fronted adjective out of a participle that takes a preposition, or else your preposition gets lost and can lead to confusion and wrong parses.
- Alienation of consumer from producer allows the existence of unaccounted for externalities.
- Alienation of consumer from producer allows the existence of unaccounted externalities.
- Alienation of consumer from producer allows for the existence of unaccounted externalities.
- Alienation of consumer from producer allows for the existence of unaccounted-for externalities.
Citations of this sort of thing in OED include:
- 1725 Pope Odyss. xxii. 164 ― Oh curst event! and oh unlook’d-for aid!
- 1817 Bennet in Parl. Deb. 340 ― He would oppose··this arbitrary, impolitic, and uncalled-for measure.
- 1842 Lover Handy Andy xlvii, ― The extraordinary capers Tom cut on the occasion, and the unheard-of lies he squandered.
- 1856 N. Brit. Rev. XXVI. 109 ― A slatternly wife and eight or ten uncared-for children.
- 1860 Ruskin Unto this Last ii. (1896) 65 ― In some far-away and yet undreamt-of hour.
- 1864 Pusey Lect. Daniel iii. 105 ― Noah was the unlistened-to preacher of righteousness during those 120 years.
- 1876 Miss Yonge Womankind xiii, ― The best endeavours··are often frustrated by some unguessed-at peril.
- 1878 Bosw. Smith Carthage 30 ― Elated by an unlooked-for victory.
- 1878 Browning Poets Croisic li, ― Who may be this glorified Mortal unheard-of hitherto?
- 1880 Geo. Eliot in Cross Life (1885) III. 406 ― The great, once undreamed-of change in my life.
- 1894 Mrs. Dyan Man’s Keeping (1899) 47 ― This unreckoned-for encounter··was a bitter pang.
- 1919 M. K. Bradby Psycho-Anal. & its Place in Life x. 12 ― An unthought-out attitude is shown and resulting unhappiness.
- 1959 O. Mills Stairway to Murder v. 49 ― Her collection of unwished-for guests.