Not available in the current interface. You can do some sql queries (via sqlite or similar) outside of LR if you want to dink with it. If you have sidecars turned on, you could also sort/grep/etc. focal length data that way (but again, outside of the LR interface).
Searching the LR forum, I see there are multiple similar feature requests in queue for this. Not sure if it's been included in the v2 beta, but I don't think so.
The trick would be to (outside of LR), pull out focal length data, then apply keywords based on or containing the recorded focal length. Then (inside LR) you could use the focal length keywords to sort just like any other keywords.
Here's the closest reference I found to a solution like that.
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