It seems that including a php file is possible (and php as you know, can include html within it no problem).
Based on this post - see
http://forum.x-cart.com/showthread.php?t=43708&page=2
So perhaps the following is better approach...
{if $page_name eq "home" && $main eq 'catalog'}
{include_php file="{$SkinDir}/{$shop_language}/homepage_content.php"}
{/if}
Hope helps a little - cheers, Asiaplay