To be fair in MOST things baking substituting some banana, pumpkin, applesauce, etc. for egg will work fine (and impart some flavor so, pick which one works for what you're doing) but in an uhh egg custard.... well... That is a bit different. (Really I would probably just use like potato starch to get something that would set correctly, but you would need to adjust flavor if you wanted to get it close since that won't give egg flavor.)