You should be able to achieve this by editing the master page to use $SPURL (relative URLs):
•Browse to your site.
•From The Site Actions Menu, Select View All Site Content
•Select the Images Library (or a picture library). Create one if needed.
•Upload your Site Logo to your sites image library.
•Open SharePoint Designer and Select File, Open Site. Type the URL of the top level of your site collection.
•Locate the master page gallery in the left pane. _catalogs\masterpage
•Double click your master page (ideally, you should have copied v4.master to my.v4.master)
•Search For: <SharePoint:SiteLogoImage id=”onetidHeadbnnr0″ LogoImageUrl=”/_layouts/images/titlegraphic.gif” runat=”server”/>
•Repace With: <asp:Image runat=”server” id=”logo” ImageUrl=”< %$ SPUrl: ~site/images/companylogo.gif %>/>
•Publish your modified master page.
What the above does is replace the default (site by site) mechanism for replacing the company logo with a hard coded reference to the company logo which you uploaded to each Sites Images library.
By doing this, all sites which inherit from my.v4.master will be updated to reflect this change.
The above, is from an article modified by me, and also not tested by me but should work, was taken from the original article by Shane Perran, which is based on SP2007, but should apply to any SP version:
http://www.graphicalwonder.com/?p=635.
Sounds scary I know, but I think its worth a shot and should work in theory. And if it does, please post back here.