I guess my remaining question, which I'm still researching, is why the Rahlfs-Hanhard Septuagint encloses the section in question (matching Romans 3) in square brackets and why the NETS felt the need to omit it completely, considering its wide textual support in the key LXX mss.
Is this done simply in deference to the MT? Or is there manuscript evidence for the idea that this section was NOT in the "OG Greek Septuagint"? The omission from NETS strikes me as quite odd.
Here's what it says in the NETS in Albert Pietersma's introduction to the Psalms:
At not a few places, Rahlfs enclosed within square brackets items of text, which, although they could not in his judgment justifiably be regarded as original, nevertheless have widespread support in the textual traditions. Since in all of these cases I agree with Rahlfs' conclusion, I have taken the next step and have excluded these items from NETS without comment.
Further improvements to Rahlf's edition have been made in the light of additional textual information (chiefly II-V CE; especially the famous P. Bodmer XXIV [Rahlfs 2110]) and more recent study. All these, however, have been included in the footnotes to NETS. Nevertheless, there remains good reason to emphasize that a liturgical text such as the Psalter, with its long and intensive transmission history, can hardly be expected to have been fully restored as yet to its pristine purity.