The EAD template that came with Oxygen has a spot for putting your
institutional logo in the publication statement, just as you suggested.
Here's a snip of it with the included directions:
<!--To link to your logo, click on the diamond in the <extptr> tag
below and enter the full URL of the digital logo file in the HREF
attribute. Describe image in TITLE attribute, eg., University of Oregon
<publisher encodinganalog="publisher"><!--Name of the publisher of
the finding aid; required-->
<extptr actuate="onload" show="embed"
<addressline><!--1st line of publisher's address.
Use as many addressline elements
as needed; OK--></addressline>
Hope that is helpful to you!