I travel with a heavy combo lock & a cable lock with loops. I do often lock my passport and money in my large backpack with this setup, locking it to a piece of furniture. I then cover this lock thing up with a towel so it's not so obvious. The only way into this combo is if someone slashes your bag. From alot of past stories I've heard, most of the time stuff stolen from the hotel is an inside job and they do it in a manner that's not leaving evidence (like taking some but not all of the money, even from safe deposit boxes, or just taking things that aren't locked). I am not a fan of the 3 digit combo locks that are prevalent for TSA, etc. The codes can be cracked within 10 minutes (I've tried it!)
When I transfer from city to city I use a money belt in case of robbing. I also carry a decoy wallet that has my $ for that day & an expired card.
On a long trip I rolled up a 100 dollar bill, wrapped in saran wrap & duct taped in a hidden compartment in my larger bag. I never had to use this but figured if someone nicked my passport & all cards that $100 might be enough to get to the embassy.
I like your ideas of $100 bill for emergency and decoy wallet.
I used to love, and utilize, tricks like this too.
Now?
Well, it's like I said in my first reply to this post, you learn not to worry about it.
99.9% of the world is totally safe. And it's very likely you won't be in the other 0.1%. Even if you're off the beaten tourist areas, you'll be very safe. Yes, be smart, don't have your money hanging out of your pocket or whatever, but the overcaution is quite unnecessary.
You're MUCH more likely to lose money to a scam, or overpaying, or that sort of thing, than you are to be robbed or pickpocketed.
If you lived in Chicago, or Miami, or whatever, and were taking a weekend trip to Disneyland in LA, would you use a money belt? If you lived in Boston, and were taking a trip to NYC to see Times' Square and the Empire State Building, would you take an extra hidden stache, or fake wallet in case you got mugged?
If so.. alright. I mean, ultimately, do whatever makes you feel comfortable.
But if you find that idea silly, as I do, it should be the same basically everywhere else in the world. Be cautious, but not paranoid. Extra precautions--beyond your normal level of comfort--aren't any more necessary when visiting Madrid or Istanbul than they are when visiting Seattle or Houston.
My point is that the world is safe, and you should get out and enjoy it, not worry about little things like the tiniest percent change someone steals your wallet.
:)