Well, sometimes I just don’t look long enough…
I found the file and code at /inc/class-custom-sidebars-replacer.php around lines 200-400.
For posts (is_single()) you go through 4 steps, 1.1 to 1.4, lines 208 to 277. Step 1.3 checks for categories.
For pages, on step 5, it’s only 3 steps, excluding the category one (lines 361 to 426). The rest is fairly similar to the posts. I now just copied step 1.3 and pasted it as ‘5.25 CUSTOM’ in between 5.2 and 5.3 and it all works!

I still got a little help request though: How do I store those changes save now; without getting replaced with your next plugin update?
Thanks for your help!

The copied code:

			// 5.25 CUSTOM If no metadata set then use the category settings.
			if ( $replacements_todo > 0 ) {
				$categories = self::get_sorted_categories();
				$ind = sizeof( $categories ) -1;
				while ( $replacements_todo > 0 && $ind >= 0 ) {
					$cat_id = $categories[ $ind ]->cat_ID;
					foreach ( $sidebars as $sb_id ) {
						if ( $replacements[ $sb_id ] ) { continue; }
						if ( ! empty( $options['category_single'][ $cat_id ][ $sb_id ] ) ) {
							$replacements[ $sb_id ] = array(
								$options['category_single'][ $cat_id ][ $sb_id ],
								'category_single',
								$sidebar_category,
							);
							$replacements_todo -= 1;
						}
					}
					$ind -= 1;
				}
			}

Hi Predag,

alright, I wont update the plugin then. Or copy it in there again manually.
Do you think you could update me (here or per email) if you integrate the functionality?

Thanks,
Jan