here's what I found in OED:

1) erogate [obs.] t.v. - To pay out, expend; to distribute.
No Man can Supererogate till he have first erogated.
thus, erogation - expenditure, esp. in the bestowal of gifts, almsgiving

2) (didn't find suberogatory, but) subtererogation, the performance of less than is required (opp. of supererogation)

3) pretererogation, nonce-wd. [after supererogation], performance beyond or outside of what is demanded or required.

it seems all of these words were pre-formed in Latin and came to us with prefixes attached, therefore erogatory would be, at best, an inkhorn term.

4) deontics [Philos.]

[f. Gr. dŚom, deoms- (see deontology) + -ic.]
A. n.

1. pl. [After ethics, eudemonics, etc.] (See quots.)
a1866 J. Grote Moral Ideals (1876) vii. 102 A science of duty (deontics or deontology). 1906 J. S. Stuart-Glennie in Sociol Pap. II. 250 The second order of ethical sciences.. form the contents of three classes of sciences—Economics, Deontics, and Juridics.
2. sing. (See quot.)
1926 Mind XXXV. 395 Ethical arguments+should be able to exhibit their ‘deontic’ in the same way as inference reveals its ‘logic’.

B. adj. Of or relating to duty, obligation, etc.