I've been told no to many upgrades to specialty cars over time (ATL to Mercedes/7 Series seems to be most often). When I've really wanted something after being told no, I've offered up extra cash to see if it works. It has both times:
1. PHX (maybe 2 years ago). They had me in a Taurus and their lot was bare. I was pushy since the website showed XTS as available (back when it was still specialty). I had our US and Korean presidents coming to spend a few days with me and I really didn't want an orange Taurus. For $25 extra, they "found" an XTS for me to use.
2. YYZ last winter. They had me in a 5 Series. They were pushy that nobody gets upgraded to a Mercedes. After haggling for a while, I settled on $20 extra for the rental, which was scheduled for 1 day. That night, my trip got extended to 5 days. When I took it back, they tried to charge me $20/day but I got it back to $20 for the entire trip