Seems to be more behind it!
I tried to include the above sample code into my theme’s spFunctions.php, and the filter gets fired. I have just put in a debug message instead of the array modification to print_r the $plugins content and it turns out to be empty! Shouldn’t there be the standard plugins in? Maybe, this is due to the ‘external’ ?
Do you really need to use an ‘external’ plugin? Essentially, the table plugin should be internal as it is uploaded to the standard plugin folder. Then just putting ‘table’ into the $plugins array should suffice. Maybe, there is an extra filter for ‘internal’ plugins?
For testing purposes, I also changed (temporarily) the install sequence in sp-tinymce-install.php adding ‘table’ to the plugins, de-activated and re-activared the plugin. Then I have put ‘table’ into TinyMEC Plugins line in admin settings and ‘tablecontrols’ into the PluginButtons1 line. But all I am getting are some ‘graphical artefacts’ in the place where the buttons should appear.

Maybe, there are some CSS issues behind that needs to be corrected to support the table buttons. Should we suspend this question until Andy is back?