The easiest way to make those register... links appear centered is to use a 1 pix. by 1 pix. transparent gif as your logo image. It then becomes small, thus, so does the table cell it resides in. That allows the table cell that those links are in to become big, and it ends up being basically centered.
On your second question, you can add tags in the HEADER field... I do that on my board right now to load the images for my dropdown menu, and it works fine.
Alternately, you should be able to edit the public_common.pl template to include your required body tags. You'll have to try to make sure that works, tho, as I've never had to do it this way...
---Skorpion