How to access CMS {content block='XYZ'} from Smarty Tags?

[sloop]

All, I'd like to access the value of a content block (and/or the default block) from a Smarty conditional tag.  I'd like this to work:

Template:
...
{content block='ShowSiteLink'}
...
{if isset($gCms->content_blocks['ShowSiteLink'])}Visit onJavascript{/if}

Can this be done?  I'd like to keep this in the template, rather than a plugin/custom tag, so that template designers can modify it later.

Thanks for any feedback.