Do I get reward (or am commanded, presumably) for honoring a grandparent? Is there some direct opinion like that out there in the Talmud ?
The Rema (YD 240:24) records a dispute on the matter, but rules in accordance with the opinion that one should honor his grandparents, just that one is more obligated to honor his parents:
הגה י"א דאין אדם חייב בכבוד אבי אביו (מהרי"ק שורש ני"ד) ואינו נ"ל אלא דחייב בכבוד אביו יותר מכבוד אבי אביו (וראיה ממדרש גבי ויזבח זבחים וגו'):
The Midrash he quotes seems to be Bereishis Rabbah 94:5, which says, in relevant part:
וַיִּזְבַּח זְבָחִים לֵאלֹהֵי אָבִיו יִצְחָק (בראשית מו, א), אָמַר רַבִּי יְהוֹשֻׁעַ בֶּן לֵוִי חָזַרְתִּי עַל כָּל בַּעֲלֵי אַגָּדָה שֶׁבַּדָּרוֹם שֶׁיֹּאמְרוּ לִי פָּסוּק זֶה וְלֹא אָמְרוּ לִי [...] אָמַר רַב הוּנָא כַּד אֲתָא רַבִּי יְהוֹשֻׁעַ בֶּן לֵוִי לִטְבֶרְיָה שְׁאֵלֵיהּ לְרַבִּי יוֹחָנָן וּלְרֵישׁ לָקִישׁ, רַבִּי יוֹחָנָן אָמַר שֶׁחַיָּב אָדָם בִּכְבוֹד אָבִיו יוֹתֵר מִכְּבוֹד זְקֵנוֹ, וְרֵישׁ לָקִישׁ אָמַר עַל בְּרִית הַשְּׁבָטִים הִקְרִיב. [...]
“And [Yaakov] slaughtered Karbanos to the G-d of his father, Yitzchak.” Said R’ Yehoshua Ben Levi: I asked all of the masters of Aggadah in the South to explain this passuk, and they did not explain it to me [...] Said Rav Huna, when R’ Yehoshua Ben Levi was in Tiveria, he asked R’ Yochanan and Reish Lakish. R’ Yochanan said that [the passuk] means that one is obligated in the honor of his father more than his grandfather, and Reish Lakish said that it refers to the covenant of the Shevatim which he brought.
I always understood this Midrash to mean that R’ Yehoshua Ben Levi couldn’t understand why it referred specifically to the G-d of Yitzchak, as if He wasn’t Avraham’s as well. The Midrash first quotes the explanation he acccepted (which I conveniently left out), and then it lists all the explanations which he didn’t like.
I assume the Rema’s proof is that while he did reject R’ Yochanan’s reading of the passuk (for whatever reason - maybe he felt it had nothing to do with Kavod?), it does illustrate that the concept exists, exactly in line with the Rema’s personal opinion.