How do I determine what the program will be called when I send it to the calculator, if it isn't dependent on the .8xp filename?

The assembled file that you attached to your post has that strange filename problem in its header, indicating that something (probably binpac8x.py) is putting that invalid name there which Wabbitemu is picking up and using.

Like Xeda said, try just having on the command line:

Code: [Select]

spasm BACKGRND.txt BACKGRND.8xp
Then resend your program.