DRIVE strength and SLEW rate in Alchitry Labs

With the Mojo v3 in the old IDE, you could adjust the pin IO standard, pullup/down resistors, slew rate, and drive strength in the .ucf file as shown here.

In Alchitry Labs, you edit a .acf file, but the only allowed keyword is “pullup” or “pulldown”. How do I change the DRIVE strength parameter in Alchitry Labs?
The relevant Xilinx 7-series SelectIO document is here on page 48.