US and UA use two different concourses at MSY, each with its own security line; I don't believe there is any way to transfer airside between them but I might be wrong on this, as last time I wanted to transfer it was over to D concourse to visit the DL SkyClub. At any rate, I have never seen a first class or elite line set up on the US concourse at MSY; if I remember correctly, it's only 2 or maybe 3 metal detectors as there's not very many gates open down this concourse. Every time I have been there, the lines have been almost nonexistent--- 5 people tops. US only operates 4 gates from MSY and one of them almost always has an RJ parked at it for the DCA-MSY trip. So there might not be the traffic to warrant/justify a dedicated elite/F line.

If the OP wants to use the UA deck's FC line he/she should check that he/she can transfer airside, and also scope out the line on the US side as it might be surprisingly empty.