If you're adding UDF's to SO lines and want them on the SO form, you will also have to add them to the SO work table. If you want them on the invoice, then you will have to add them to SO Invoice Detail and then the SO Invoice work table. (It's really not that hard once you do it.)
Finally, if you want them in AR invoice history, you should refer to the post you mentioned.
https://90minds-com.socialcast.com/messages/14914462.