It's possible to pass a property to a lexicon tag:
[[%lexicon_entry? &placeholder=`foo`]]
which is then available as [[+placholder]] in the lexicon value. Similar constructs exists for other ways of getting a lexicon key, like in the manager javascript:
_('lexicon_entry', {placeholder: 'foo'});
and php:
$modx->lexicon('lexicon_entry', array('placeholder' => 'foo'));
Depending on how you're using it, you could pass a system setting via a property. By default they're not available.
That said, if you're using the [[%tag]] approach in a template, the lexicon could contain setting tags [[++site_name]] which will then be parsed in the next parse cycle, so then it does work.