Leolaia
A few verses earlier than Joel 2:32 LXX, Paul would have found such expressions as to onoma kuriou tou theou humón "the name of the Lord your God" (v. 26) and egó kurios ho theos humón kai ouk estin eti plén emou "I am the Lord your God and there is none beside me" (v. 27), and similarly in 3:17 LXX he would have read the "Lord" declare that he is kurios ho theos humón kataskénon en Zión orei hagió "the Lord your God encamping in the holy mountain of Zion". This is somewhat similar to how Paul uses Isaiah 45:23 in reference to Christ in Philippians 2:9-10, despite the presence of ton theon in the LXX version of the verse and the declaration two verses earlier that "I am God and there is no other" (egó eimi ho theos kai ouk estin allos; 45:21).
It is quite unlikely that Paul (or other NT writers for that matter) would draw such quotations from a reading of the full text of either Joel or Isaiah, even in the LXX. He too was quoting "verses" either from memory or from testimonia (or from memory of testimonia). So the actual OT context of such excerpts has little bearing, I'm afraid, on his interpretation.
Of course the Septuagint equation of kurios (especially anarthrous) with theos was common knowledge for LXX readers/hearers and it can be found in the NT (especially in LXX-like passages such as Luke 1--2, where the NWT substitution of "Jehovah," although factually wrong, does not alter the meaning). But reviewing the Pauline uses of kurios it strikes me that Paul avoids it. To him the Father is theos and the kurios is Jesus. Against Johannine or post-Pauline literature, or Hebrews, he doesn't toy with the idea of calling the Son theos, and I fail to see any unambiguous instance where he would call the Father kurios. Perhaps in the full quotation of Romans 14:11, but then it is not in a christological argument -- and when it is, in Philippians 2:10, theos leaves way to Ièsous and kurios.