I think you might get some use out of making a custom document type and using a one-record repeater to display the HTML.
Make a new doc type and call it "Shared Content"
Add field:
- Content (long text - html editor)
Then put your shared blocks in a folder of their own and manage them separately from the rest of the content using the Form tab for the document.
To use them, insert a repeater web part on the template you're working on and choose your Shared Content doc type, and select the exact doc you want to use in the Path property. That'll ensure only that one gets listed. Then set the transformation to a very simple one like the following:
<%#Eval("Content")%>
I had this problem a while ago so I programmed a custom web part to allow me to select from a list to very easily include shared html content on any template. Video here:
http://www.youtube.com/watch?v=IfSVIf72u7g