Yes, there's a whole separate set of non-4-digit alt-keypad characters. Codes 1 to 31 work without leading zeros but not with them, codes 32 to 126 produce the same character whether entered with or without the leading zeros (but they're on the keyboard, so who needs them?), and 127 and up produce different characters if preceded by a zero than if not.

Bricscad can produce Unicode characters, with that \U+hexcode trick. And it can display any character that was created in another app by alt-4-digit coding. I can create a theta (Ɵ) in Atlantis by pressing alt-0415 (though, interestingly, here in the message form it creates a Y-umlaut Ÿ, which would be created by alt-0376 in Atlantis), and then paste either of those characters into Bricscad successfully.