Thanks very much for your reply. To be clear you probably don't need a Steam Deck to test whether it would work on the Deck — any PC with a common Linux distro (Mint, Ubuntu, Bazzite, etc) with Steam installed and working should be a suitable test platform. I think the key questions to answer are simply: "Can I add SQ1 to Steam as non-Steam shortcut?" and then "When I try to launch SQ1 through Steam, does the game start?"
I wish I could be more helpful but even though I've been a heavy Deck user for nearly 4 years now, I still have only an elementary understanding of Linux. Such is both the greatness and weakness of the Deck that it can be thoroughly enjoyed in many different ways without ever needing to actually learn how it works.