Is it possible to debug PhpUnit tests with --process-isolation option?

For unittest

class SampleTest extends PHPUnit_Framework_TestCase
{
    public function testBreakpoint()
    {
        $a = 18;
    }
}

with breakpoint on line 5 "$a = 18;",

• Xdebug v2.1.0,
• PHPUnit 3.6.10,
• PHP 5.3.6,
• ubuntu 10.11

Running unittest with NO --process-isolation option stops script execution on the line 5, as expected. Running the same configuration WITH --process-isolation option does not stop execution on line 5.

The option --process-isolation runs every test in new process using 'proc_open' in runJob function in https://github.com/sebastianbergmann/phpunit/blob/3.6/PHPUnit/Util/PHP.php

Tested with PhpStorm 3 and vim 7 with debugger plugin. It allows to debug PHPUnit itself, but not testcases.

Is there any way to debug the child process created by PhpUnit using Xdebug? may be Zend Debugger?