Can I access semantic configuration in a compiler pass?

Yes, kind of:

<?php

namespace Acme\DemoBundle\DependencyInjection;

use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;

class CompilerPass implements CompilerPassInterface
{
    public function process(ContainerBuilder $container)
    {
        $configs = $container->getExtensionConfig('acme_demo');
    }
}

From what I can see $configs is an array of unmerged configurations and default values are not included (values defined by the configuration TreeBuilder).

See here and here

Just for the sake of completeness to @Peter's answer: getExtensionConfig returns an array of arrays which should be processed with the corresponding Configuration to be able to access default values.

<?php

namespace Acme\DemoBundle\DependencyInjection;

use Symfony\Component\Config\Definition\ConfigurationInterface;
use Symfony\Component\Config\Definition\Processor;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;

class CompilerPass implements CompilerPassInterface
{
    public function process(ContainerBuilder $container)
    {
        $configs = $container->getExtensionConfig('acme_demo');
        $configuration = new Configuration();
        $config = $this->processConfiguration($configuration, $configs);

        /// You can safely work with $config now
    }

    private function processConfiguration(ConfigurationInterface $configuration, array $configs)
    {
        $processor = new Processor();

        return $processor->processConfiguration($configuration, $configs);
    }
}

I realize that this is an old post, but I was looking for the same information and eventually found that this works for a single parameter:

$cfgVal = $container
  ->getParameterBag()
  ->resolveValue( $container->getParameter( 'param_name' ));

Of course it's possible that this functionality was added after the original post.