As a one-off the first time, it was inspired.
Yes, I think so too. It reminds me of the whole padlock thing, where some couple (in Paris, was it?) left a little poignant symbol of their love, with the two of them locked permanently together. One little padlock down in the corner was quite sweet. Fast forward a little while and then everybody is doing it everywhere. That bridge in Bakewell nearly collapsed under the weight. I remember seeing one of them that had been engraved with (I forget the exact first two names but e.g.) 'Tom & Chloe 4 Eva' and wondered if they realised that it made them look polyamorous 
So many great and witty ideas are completely spoiled when other people copy them in their thousands. If everybody does want to copy, I wonder if they still would follow suit if somebody put up a big sign saying something like "I have donated £30 to the food bank in memory of HMQ" - I mean whether they would donate, rather than just put up their own sign without giving any money....
As for setting up a specific charity, it's a nice idea in principle, but as well as the huge amount of time and resources it takes to do that, I think it would rather give the wrong impression when contrasted with the sheer amount of wealth that the RF has. I always think that, if I were a billionaire and somebody asked me to be the patron of a new charity that they wanted to set up - one which was close to my heart - I'd tell them not to bother with all of that faff and just fund it all myself.
I suppose something like the Prince's Trust is a bit different, where you're actively seeking to influence/inspire others and promote schemes rather than just asking for money to support those whom the government and/or society have failed.