Ivan Vucica gave me an idea in a post on the Frontier-user list.

He suggested using ShellExecute. I did a search, and it seems that's the way you do it on Windows. I don't want to touch the kernel source, but I can easily search it. So I did a search for calls to ShellExecute, and lo and behold, that's how launch.application is implemented on Windows. So I tried creating a shortcut file with the winShell verb that does that, and just threw it over to launch.application, and goddam if it didn't open in the default browser, which is what you want.

Moral of the story, use every bit of information available to you, if possible.

local (f = frontier.getsubfolder ("/ops/tmp/") + random (0, infinity) + ".url")

winshell.createurlshortcut (f, s)

launch.application (f)

file.delete (f)

webbrowser.openurl