1. I tried to use a format-string directly instead of lfmt pointing to a function, but when I tried that I got the message:
apply: Not enough arguments for format string
Through some experimentation, you can find that the format string is only receiving one argument, which is the list of other arguments. I'm guessing this is a bug in orgtbl-apply-format, and that (apply 'format fmt args) should be (apply 'format fmt (car args)) or something else.
2. Even if you define your own exporter function, you can't use org-table-export, because the list of exporters are hard-coded. That's fine, but in that case, can you add an example on how to use the TABLE_EXPORT_FORMAT and TABLE_EXPORT_FILE? Just something like the following in the "Built in table editor" section would have made my life easier.