Any one ever modified the ButtonBar add in to include the upload image option?
I believe lincoln did, at one time, but not via this plugin,
in this link he mentions something about new editor, which may have the feature you want.
as far as adding buttons to the buttonbar, this may give you some insights on how buttons can be added to buttonbar and you can adapt or modify to meet your own needs for your_ flavor_ of vanilla. The way to approach it might be different depending on your flavor.
I may not provide the completed solution you might desire, but I do try to provide honest suggestions to help you solve your issue.
I am not sure of the reason why anyone would want to though or what value it might add, since the existing image upload plugin works perfectly adding an icon to the button bar area, and seamlessly.
the following would require changes in both plugins.
but yes I tried it as an experiment and it worked for me with my flavor of plugins and vanilla.
should you desire to experiment this is what worked for me. It may or may not work for you.
it is purely an idea on how you might experiment and go about the change. you may find a better way.
just add the info from upload_button.php
put it in a span with class="ImageUploadWrapper">
and insert it within the div of buttonbar.php properly.
then comment out the echo $Sender->FetchView for upload_button.php in the image upload plugin and remove the float left in the css.
that said, if the above hints work for you great. if not, perhaps someone else can provide a way, but as the 2 plugins exist today, it just requires more changes without a true added value. Since one would have added no functional difference, per se, but just swapped around the deck chairs.