You might be able to add something to a Master page so that it is in the top right of each topic. Not quite what you wanted but close.
The other solution is to add the build number to the Title Bar, first field in the WebHelp settings. Quick and easy and always visible.
See www.grainge.org for free RoboHelp and Authoring information.
I'm wondering if the built in About box will work?
Look at your Single Source Layout settings for WebHelp.
Specifically, click the Navigation item on the left side of the dialog. Then ensure the About item is enabled.
This should then enable the button so it appears in the main toolbar across the top of WebHelp.
You are able to choose the image that appears there.
When clicked, your user will see this:
And you are able to change many aspects of this. Just head to your Output Setup pod, open the Skins Editor and double-click the skin you are using. Then change the About images and information.
Perhaps this will get you closer to what you were hoping for?