I'm sure I'm not the first to do this but it's first time I've tried and thought I'd share..
I was messing with light to adjust it one day and found a good spot - mounted upside down. Added bonus that it's less conspicuous this way too as it no longer protrudes (sometimes, I take a chance and leave light on the mount after I park and I've had a couple stolen). Me thinking this is less likely to walk away. Plus, it's not as natural to dismount it this way so I'm thinking the type of person that steals bike lights might give up pretty quickly if they can't get it off in a hurry.

That's fine for you, as your light has a round beam. Don't do this if your light has a shaped beam.

almost all current cygolites have a symmetric beam that isn't round. It's the same upside-down as it is right side up. They use a Fresnel lens. I forgot that one time on an undulating road where it would have been nice to have a little light higher up. Turned on the cygolite and got nothing extra because it does have a pretty good vertical cutoff. One of the few battery lights that does have a decent vertical cutoff. Now I have it aimed up a little and use it for descents sometimes.

A fresnel lens can also aim the beam by the way, it just depends on where you cut it. Something like this https://www.amazon.com/OpticLens-Fle.../dp/B00ET4936A (not necessarily that one, but for example), just cut out a piece in the same shape as the light and slip it inside the light's lens cover. In the center for symmetrical spreading out, a piece from towards the edge will angle the beam. Some light will be dispersed or blocked of course, they aren't perfect. Just an FYI

I tend to mount all my lights this way, underneath the handlebar or on the stem underneath, because I like them out of the way.

my dyno light can be mounted upside-down. I chose not to do it though. Probably next mount I make will have it upside-down.

Many dyno powered lights should be mounted right side up. Not sure why but many of the dynopowered lights have a vent hole on the bottom that could collect rain if you mount it upside down. So, inspect your light closely or maybe do a pertinent google search to see if it has a hole that is supposed to be on the bottom, not top.

I have not noticed vent or drain holes on battery powered lights, but if a light had a lot of waterproofing seals it might also have a drain hole to help equalize pressure.

I recently bought a dynopowered taillight and was a bit dismayed when I saw it had a vent hole on the bottom, if I mount it on a rack without a fender I am sure much of the tire spray will go upwards towards the light.

Yep, that's one of the problems that all of us have. Especially when you happen to use a lot of stuff on your bike that just so happens to be very easy to remove. Bad enough you have to worry about if someone wants to steal your bike. But if you have a bunch of other stuff on your bike that would only take a couple seconds to remove than that too becomes a problematic issue, even if you only want to leave your bike somewhere ( out of sight ) for just a couple minutes.

I carry a little cable lock in my frame bag that I use just so I can go into a convenience store and not worry about the chance that someone might quickly grab my bike in a " casual thief of opportunity" scenario. Of course if I wanted to go into a restaurant and eat then the issue changes, especially if I can't see my bike while inside the restaurant. In that situation I would almost feel compelled to remove everything that is on a quick release. I have a lot of stuff so that would be a total PITA.

Anyway, if I had to do that I'd take off my QR lights and the protective sleeve that holds my phone and then use my helmet as a bucket to hold the stuff. Still with the flimsy cable lock I use ( which is very lightweight, slim and minimal ) a person with a set of wire clips could have my bike in seconds. That's why I won't go into a restaurant and eat unless I can see my bike while I'm eating. Not to me mention that if someone steals your seat / seatpost that would be an even bigger PITA. ( thankfully my seatpost isn't on a quick release or I'd have to take that along too or use a separate cable lock just for the seat ).

Yeah, I do exactly the same thing. I use my wire ski lock since it's lightweight and adjustable and often just leave it on the panniers. So, as you mentioned, someone that really wants it could probably cut thru it with a pair of toenail clippers but anyone just walking by probably would give up after realizing that it isn't a quick grab (as the saying goes, the kind of lock that keeps out "honest" people).