That's a valid and important concern, and one that I don't have any answers to right now. One possibility is keep capital fixed while making people mobile, i.e. very tight capital controls combined with very loose immigration controls. Capital thus cannot escape to low-tax regimes, and people in developing countries can take still advantage of capital by moving to where the capital is. If that's too difficult to handle, there is also the possibility of "virtual" immigration through political union.
> You'd have to grow your capital at a rate greater than the tax every year to be successful
Since the tax would be about the same as the current (or 1960s, if you're generous) share of the national income from capital captured by wages, capital already grows at at least this rate--otherwise wages alone would be dragging down all economies today, and that's just not happening.
> What if your whole economy had a bad decade and assets failed to outgrow your capital tax?
We'd do what we do today, and draw down the sovereign wealth fund and/or borrow from the bond markets.