I'll double check with both of these, but perhaps things have changed recently. Thank you.

The latest HHonors car rental rewards page (http://www.hilton.com/en/hhonors/rewards/car.jhtml) makes no mention of SixT.

I can find a Priority Club page that allows points to be earned for Hertz rentals in Europe (http://www.ichotelsgroup.com/h/d/pc/1/en/c/2/content/dec/pc/1/en/partners/europe/carren.html), but the only rewards I can find are limited to the US (http://www.ichotelsgroup.com/h/d/pc/1/en/c/2/content/dec/pc/1/en/points/us/rr.html).