I've made a plugin that removes (most of) the line breaks, though, so that's a start.

Turns out I was able to hack around enough and get a plugin that works on my system, at least. I've released it as a version 0.1 plugin named BetterWPReadMore.

The way to fix this (so that a WP-upgrade doesn't undo your work) is to use the WP filter system.

Only some of this can be done via a plugin. Variables like $more_link_text are never in scope, so I have no way to easily determine the link's text (<a href="http://permalink"></p><p class="readmorelink">Read More</a>), so I have no way to search for that string and replace it. I could probably do so with preg_replace, but that approach seems quite easily broken since $more_link_text can contain HTML.

I've made a plugin that removes (most of) the line breaks, though, so that's a start.

add_filter('the_content', 'nslog_read_more', 7);

You can see where filters registered to the_content are applied by looking for the apply_filters(); call for the_content Note that the name the_content is meant to correspond to the_content() but that doesn't mean that every function automatically has a filter.

I have a quasi-documented list of filters here.

The nslog_read_more() function should take a string input (it'll get the content of the post) and should return that content (after modification, of course).

The third param (7) is a priority for the filter. Default priority is 10. I've set this to 7 because at 10, wpautop() kicks in. So for the preg_replace() calls in nslog_read_more(), you'll be dealing with linebreaks, not paragraph/break tags. If you wanted to deal with the HTML, you could hook in using a priority of 11.