Yes, that is what Warren Buffet did, if I recall correctly. He reasonably surmised that he was excellent at investing his money to get good returns, therefore he should get the good returns whilst he could before he gave the money to charity (I think he is donating to the B&MGF).
If you took all people with personal wealth over $5b, and did a one-off tax of 25% of wealth between $5b and $10b, and 50% over that, you would have a one-off tax income of $150b, but only affect 57 people. Lower the former boundary to $1b however, and the receipts would be far higher, yet you would still affect under 400 people. You could run the US military for a few months on that! (puts it into perspective, eh?)