I haven't been to Funston Lake, but have, on two occasions, been to Rocky Lakes Basin. Both times I travelled over Cottonwood Pass down to Big Whitney Meadow and then picked up the trail to Rocky Lakes Basin. While camped at the lakes the first time, I took a cross country route out of the lower lake to the ridge which looks down on Siberian Basin. From there I continued my climb up the attached ridge and followed it for about a mile (the ridge is amazingly flat). The ridge looks down on Big Whitney Meadows to the south, Langley and the Major to the East and Whitney to the North. Below was Siberian Basin.
I am very fmiliar with Siberian Pass and the surrounding area having hike the pass and sections of the PCT.
The next day, I again took the route up to the ridge and decended down to Siberin basin and followed the basin East until I intersected with the Siberian Pass Trail.
The sectond time, I repeated the route except that about half way toward the Siberian Pass trail, I went North since I knew from looking at the maps that the PCT was about 200 yards in the trees. And so it was.
Going back to the first time, while on the ridge I explored other ridges, but not Boreal Plateau.
So, besides the Rock Lakes Trail, there are no other trails. However, your thought of taking the route through Siberian Basin is reasonable.
I will mention this to you, if for some reason you take the Rocky Lakes Trail I decided to tae a cross country route up a canyon to the lower lake. From Cottonwood Pass Trailhead to Lower Rocky Lakes it took me about8 8 hours.
To be honest I have been interested in Funston Lake and have heard over the years of cross country route down to the Kern where it is about a mile from the hotsprings.
Oh, all the fish have been removed from Rocky Lakes back in 2008. No fish!!!!