Catrina: thanks for your advice, but I am already using custom CSS. But that doesn't solve my problem of adding "By Author" only to some posts. Since you can't add span (with classes or ID) directly to the metabar, CSS can't do that.
But I managed to find the solution myself, and it is elegantly done by Pagelines. They just forgot to tell us about it.
Looking into the code I realized that I can actually add information to the shortcode like this
to do what I want. This is documented anywhere else than in the code unfortunelately. Please do!
For reference for others looking for help:
The shortcodes in the metabar accept the following parameters, before, after and sep. Sep indicates what separator to use between tags and categories. Default is ", " but you can change it to anything you like. I use
Before and after puts the text before and after the shortcode. Most of the shortcodes for the metabar seems to accept before and after. By experimentation I found that you are better off using non-breakingspace than just a space, as spaces may be removed by Wordpress.
For dates and time I guess you can edit the format by using somthing like format= with PHP-date string if you want something else than the default format, but this isn't explained in the code. There's also a label code, but I don't know what it does.
Pagelines add classes to the shortcodes, so you can target the different pieces of the metabar to make them look differently, and hide them on some pages. I use the following code to only show author in one category archive: