Vanishing withholding tax
Many jurisdictions allow deducting losses from gains if you are a tax resident there. Whilst there might still be withholding taxes, they will be returned in full if there are no net gains. E.g. in Switzerland legal entities generally work like that.
Let’s say we are a company (or some more elaborate legal construct). We want to get the risk free rate and some (potentially heavily arbitraged) extra for our market making. So we sell some future on a gross total return equity index (e.g SIM TR) and buy all the underlying stocks. The price movement and returns will cancel out. No gains there and no tax.
The remaining cost of carry consists only of borrowing rate (approaching risk free rate), storage cost, and convenience yield. Storage cost and convenience yield should be very low for such stocks. That is some cost for buying, selling stocks, and rolling futures on one hand, and some yield from security lending on the other.
Which leaves about the risk free rate. The market will have to pay just this through the futures price. Now, since we are are an elaborate legal construct with a risk free business, we borrow endless money from someone else to make some asymptotically small arbitrage gain. We could also use the borrowing cost to transfer any gains to some tax heaven.
The twist
Ok, but we don’t have enough capital to pull a multi-billion transnational organization from our hats. Also we don’t want the risk free rate, but the gross total return of an equity index. True, but that is not a problem, because global capital will do all of the above for us.
We just buy that futures they sell and get the gross total return of the equity index minus the risk free rate. Capital gains from futures are normally not subject to withholding tax. We then pay our local taxes on the gains. In Switzerland we would do so and pay taxes inside a legal entity. Else the taxman will declare you a professional securities trader before you can blink.
But what do we do with that capital that we don’t have to borrow? Buy Irish bond ETF to offset the risk free rate on the futures. No withholding tax there on any level.
tl;dr: Theoretically you don’t have to pay any withholding tax.
PS: Feel free to compile some empirical data.