OK I see there's no reply so far. Here is the best workaround I came up with so far, using free software:
1) Install Oracle VirtualBox
2) Install an up-to-date version of Linux on VirtualBox (I used Debian)
3) Install F5 client for Linux on the VirtualBox
4) Connect to the VPN through the VirtualBox
Of course, you could also partition your hard drive and install Linux there, but the VirtualBox is more convenient, and doesn't require rebooting or exiting Windows 8. (I've also had difficulty in the past booting into another OS because of Windows 8's Secure Boot feature... maybe I should start a separate thread about that).
Still, it would be more convenient, and simpler, to just run the F5 client directly under Windows 8, but I still don't know a way to do that.