What do you mean throw in N and apply the brakes, lol WTF....??????
If she's going down hill the transmission can be manually shifted up and/or down on command, even if you didn't go downhill, you can shift any modern trans up/down as you wish. And it'll be better if you selected the proper gear to do some rev matching and hold the truck weight with both brakes and trans when going downhill. You're not going to damage the trans because of it. If it happened i'll get rid of the truck as quickly as possible and badmouth the vehicle for having such a chitty trans everywhere i'll go. This are not the 60's anymore. Auto trans are in many many respects a lot better than the best manual trans we ever had. I still love a manul trans don't get me wrong and i know how to kung fu kick the clutch when needed as well lol, but auto trans are just damn impressive now days, especially the ZF8.
If the OP really wanted to tackle this as a programming issue, the answer could be as simple as changing the shift points with HPtuners. i've looked in the tune for both trans though in a couple of Hemi 1500's including mine, 6 and 8 speed, and i don't see a parameter to define shift points based on engine coolant or trans temp per se. I see tables in the 6 speed trans for super cold shifts and cold shifts that could define this type downshifts , but i don't see the table that should define what a cold/super cold shift actually is. There are for example 4-2 or 3-2, 2-1 downshift tables for normal, cold and super cold, snow etc modes, i don't see the same tables for the 8 speeds, at least not with the cold/super cold definition. Could be there though i'm not that proeficent with the Mopar transmissions just yet. I did change some of the shif points in my 6 speed to better match the 37's i have to the 3.92 gearing for 2-3-4 gears as the factory shift points are lazy in the 6 speed. As for the 8 speed i'm still learning, i have an 8 speed trans in my Durango and i'm still studying how to tune that ******* trans