External file without using iFrame

Please confirm if I have understood correctly.

This local file will be included
[includeme file=”includes-test/hours-of-business.html”]

This remote file will NOT be included
[includeme file=”http://remotedomain.com/includes-test/hours-of-business.html”%5D

This remote file inside an iFrame WILL work
[includeme src=”http://remotedomain.com/includes-test/hours-of-business.html”%5D

If I am correct, is it possible that you could upgrade the plugin to include external files without the need for an iFrame or is there some technical restriction with WordPress that makes this not possible?

Thanks ColinK