Hi,
Well, I don't see why it wouldn't work.
Maybe the regex needs to be adapted because of the way the text in the description is.
Maybe try to add the flag "s" to your regex:
echo JHTML::_('content.prepare',preg_replace('#.*<hr *id="system-readmore" */>#is','',$this->element->product_description));
Alternatively and simpler would be to use a custom product field to add the images on the right side of the product page.
That way on the product edition page, you'll have 2 distinct fields, one for the description, one for the right side images area.