Passive Morphemes in a Passive-less Language?

In his comprehensive and succinct overview of passives in the world’s languages, Edward Keenan (1985) makes the following typological observations. First, not all languages have passives; and second, such passive-less languages commonly use one of the following grammaticized means to express functional equivalents of passive: (a) to use the “impersonal” third person plural subject; or (b) to eliminate the subject of an active construction. Tongan is cited as an example of the latter, but with a disclaimer that “[i]t is not clear whether we want to consider such cases [as (1b)] ... as a ‘truncated’ active, with perhaps a third person plural or indefinite pronoun understood or as some kind of morphologically degenerate passive in which the verb form is not distinctively marked” (Keenan and Dryer 2006: 330).