I haven't, but I like the idea.

I don't stamp my money, but did come across this not long ago while researching the history of our nation's de facto motto- E Pluribus Unum, and that stamper was one of the search results that came up.

Here's a link to the Flying Spaghetti Monster stamp [fsmstamps.com]

It is illegal to write on US paper money or coins .

[lawyers-plus.com]

"Whoever mutilates, cuts, defaces, disfigures, or perforates, or unites or cements together, or does any other thing to any bank bill, draft, note, or other evidence of debt issued by any national banking association, or Federal Reserve bank, or the Federal Reserve System, with intent to render such bank bill, draft, note, or other evidence of debt unfit to be reissued, shall be fined under this title or imprisoned not more than six months, or both."

"Whoever fraudulently alters, defaces, mutilates, impairs, diminishes, falsifies, scales, or lightens any of the coins coined at the mints of the United States, or any foreign coins which are by law made current or are in actual use or circulation as money within the United States; or
Whoever fraudulently possesses, passes, utters, publishes, or sells, or attempts to pass, utter, publish, or sell, or brings into the United States, any such coin, knowing the same to be altered, defaced, mutilated, impaired, diminished, falsified, scaled, or lightened—
Shall be fined under this title or imprisoned not more than five years, or both".