i was able to get the following to work, for anyone looking for a small gist:
const dep = b.dependency("mytool", .{});
const install_step = b.addInstallDirectory(.{
.source_dir = dep.path("."),
.install_dir = std.Build.InstallDir{ .custom = "tools" },
.install_subdir = "mytool",
});
exe.step.dependOn(&install_step.step);