Rav Moshe Shternbuch in his responsa, while discussing a question whether wearing "short" suits, which were a modern advent, falls into the prohibition of Chukas Hagoyim, writes a general principle regarding the prohibition: "The law of Chukas Hagoyim only applies when there is no rhyme or reason to the custom. But if there is a practical purpose the custom, it is not prohibited"
Following this line of reasoning, sitting in the manner of the Romans, is not impractical. On the contrary it was more comfortable, and was a considered a dignified way to sit.
Therefore, on Seder night, when we are actively trying to present ourselves as royalty, aristocracy, and men of freedom, this way of sitting is not a prohibition of Chukas Hagoyim.
However this does not imply that it is prohibited throughout the rest of the year, as it is legitimately a comfortable way to sit. Thus it is not a problem of Chukas Hagoyim.