I managed to get Google +1 work with this plugin with the following custom button:
<div class="addthis_toolbox addthis_default_style ">
<a class="addthis_button_google_plusone" g:plusone:size="medium"></a>
</div>
Which works just fine.
However there is one minor thing. When I go to AddThis Plugin Options for the second time the custom code has no spaces between tag attributes, so it looks like this:
<div class="addthis_toolbox addthis_default_style">
<a class="addthis_button_google_plusone"g:plusone:size="medium"></a>
</div>
If I "Save" it then again, I get:
<div class="addthis_toolbox addthis_default_style ">
<a></a>
<a class="addthis_counter addthis_pill_style"></a>
and the button goes away.
Same thing happens with other custom code (E.g. FB Like)