Can I Reverse/upside-down mount SMD LED
Depending on the package, this can be done.

For my work, we evaluated several types of LEDs, and being mountable "through-PCB" was mandatory.

The right LED shown in the picture can be mounted normal and through-PCB, the second is exclusively for through-PCB, and the LED in the middle is a standard and very common 0603 package, not foreseen for through-PCB mounting.



It turned out the type in the middle had the best electrical/optical characteristics, and its small size was a benefit for us, too. 

The package is a piece of ceramics with metal contacts of almost the same size on top and bottom side. As there are re-reeling companies out there which will turn the LEDs in the reels upside-down, it was possible to machine-place it like any other component:



Just note that size and tolerance of that transparent resin is not that accurate, so add some extra tolerance for the hole.

About possible temperature issues: During soldering, probably the entire tiny 0603 package will heat up to the solder temperature, so the resin and the LED chip should resist solder temperatures for a short time. Due to the size, I doubt that the resin will cool down faster when mounted normally. But I would recommend to solder at the lowest possible temperature and as short as possible. Also, have a look at soldering recommendations in the data sheet. 

In general, no guarantee can be given, but it worked for us quite well.