You can create an elliptical coordinate system with PAR1 and PAR2 of the LOCAL command. I've never used an elliptical coordinate system to select entities, but you could try with a simple test model:
csys,12 ! elliptical coordinate system
nsel,s,loc,x,value ! hard to say what this will do.
esln,s,1 ! select elements with all nodes selected
allsel
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