I don't really like the "B" for Build, because it would be the same for "Build Language" etc. On the other hand, if it's only available in the service.xml editor, it might be an option. But Eclipse is all about building something...
My default binding on Linux contains two-stroke key bindings for the editor, e.g. "Format Source" is triggered by "ESC Ctrl-F", where ESC actually opens a small tooltip showing the available options "Format Source" and "Toggle Comment"
It seems that this is linux-only, but if it's available on other platforms as well, I like it because of the additional hints given in the tooltip.
The old editor actually was not using eclipse best standards. Source views should try to give as much real estate to the actual source code at least that is the best practice. As a workaround you can right click in the source view and use the "Liferay > Build Services" action.
The best solution may be to register a keyboard shortcut for when the service builder editor is open. How about
Ctrl + Shift + B or Ctrl + Alt + B