ἥρως
This term often is used as the sixth foot in a line (over twenty times in the Homeric epics), and is used as a vocative (in any metrical position, but most often in the first or sixth foot) four times in the Iliad and four times in the Odyssey, either alone, as here (see also Iliad 20.104, Odyssey 4.423, 7.303, 10.516), or with the personal name of the hero (Iliad 11.819 and 11.838 with Eurypylos and Odyssey 4.312 with Telemakhos). So although Hainsworth (1993:194), commenting on this line, says that “the unqualified vocative in the sixth foot in mid-speech is otiose and rare,” isolating this usage with that many qualifications may be overstating it. The equivalent nominative form is often found in the sixth foot, and there is even a fairly common formula that uses it in this position, αὐτὰρ ὅ γ’ ἥρως (Iliad 5.308. 5.327, 8.268, 11.483, 13.164, 23.896), indicating that this word in the sixth foot was useful and good for composing-in-performance. The other vocative uses in this position that appear in our texts, it is true, are those in which ἥρως modifies a name, but since the vocative is used elsewhere, to find this particular use strange may be a case of already concluding that Iliad 10 as a whole is strange, and using this as one more piece of evidence for that conclusion. From a compositional point of view, a word often used in the sixth foot could easily be used in this way by analogy.