Leveraging private capital If an ultra high networth individual, like Bill Gates, Mark Zuckerberg, Carlos Slim, Paul Allen, Elizabeth of Windsor, Amancio Ortega,  Liliane Bettencourt, Stefan Persson, Bernard Arnault etc. would leverage their fortune in the way a bank does with savings, would that multiply their wealth?:

If an UHN individual owns their own 'bank' and puts their fortune in it as savings, they could create e.g. 10 x their fortune in loans.
Its legal to borrow and lend money, but if you become anything close to a bank, (take deposits, make loans, take risks, make a business out of it) then you will be regulated. You will be insured by the FDIC, and since you are insured, you will then be required to follow certain guidelines regarding how you asses risk and which risks you can take or not. Moreover, they will not be allowed to lose money in a large scale, so they would have to recapitalize the 'bank' after any of the charitable projects swallows up capital.

So to answer your question, they would not have more wealth, but they would manage a bigger pot of capital with which to invest. However, at some point they will lose the freedom of doing whatever they want. Since its not their money, then they can't do whatever they want.