Completely untested but you *might* just be able to add them to the filters :

			$BlogBList->set_filters( array(
					'authors' => $author,
					'ymdhms' => $m,
					'week' => $w,
					'order' => $order,
					'orderby' => $orderby,
					'unit' => $unit,
					'limit' => 999,
					'cat_array' => array( 1, 2, 3, 4 ),
				) );

¥

I went slightly different at it. Above the BlogBList->set_filters thing is another thing.

			<?php
			$BlogBList = & new ItemList2( $Blog_B, $timestamp_min, $timestamp_max, $posts );

			$BlogBList->set_filters( array(

So make stuff be what you want...

			<?php
			$posts = 999;
			$BlogBList = & new ItemList2( $Blog_B, $timestamp_min, $timestamp_max, $posts );

			$BlogBList->set_filters( array(

But I haven't done it to my actual linkblog in the sidebar yet.

After a couple of days fiddling around I got things to work. I made two skins, for two blogs. One serves as a front page only (based on multiblog) and the other has the actual stories. The design is based on, ahem, my favorite cigar brand...

Go have a look: http://www.ornstein.org

And thanks for the help here.