Passing PHP objects and methods as strings to FormAPI
I think your best bet is to call a function that will call 

$myObject-&gt;_my_after_build_function();


One way that won't work most likely.  

$method_name = _my_after_build_function;  
"$myObject-&gt;{$method_name}"


Another shot in the dark that might work if using PHP 5.2.3+ http://php.net/manual/en/function.call-user-func.php.  

"myObjectClassName::_my_after_build_function"


You can try the above 2 methods but with a &amp; in front of it. http://www.php.net/manual/en/language.references.pass.php