I've come to the view it should go in. I think Rick's case is a good one, and I see lots of examples of it being used as a free-standing "awesome". Also there are plenty of 形容動詞 cases such as "現行ライダー恐るべしな者です。" which doesn't get handled well by the text glosser.

I could go either way. Certainly the refs only have 恐るべき, and examples in Eijiro and Tatoeba also. Yet Google claims 恐るべし has 2.5 times more hits. Doesn't make sense.

i don't think this should be an entry; べき and べし are alternate forms of each other.  we have 恐るべき and entries for 恐る and べし.  all refs have 恐るべき, none have 恐るべし

Found in Takemiya Yuyuko's "Golden Time 2", page 151.  I asked A. B. Tsunezawa, a translator recently moved to my area, and he replied:

恐るべき and 恐るべし sound almost the same as a meaning, but different as a function.
恐るべき is an adjective, whereas 恐るべし is not. Like other adjectives, 恐るべき will be followed by a noun, but 恐るべし can be placed in a sentence by itself.
 
Also 恐るべき can likely be meant negative such as "scary", whereas  恐るべし can be used to describe something in a positive way.
 
It reminds me of this English word "awesome," which originally should mean scary or dreadful. (But it won't be used as "The party was awesome.")
 
恐るべし can be used for someone who does (or did) something extraordinary.
 
 
Example 1 恐るべき
That 恐るべき criminal has run away from the prison and is still somewhere in town. 
(That "dreadful" criminal has run away from the prison and is still somewhere in town.)
 
Example 2 恐るべき
We need to prepare ourselves for the coming 恐るべき (dreadful) ID theft age.
 
Example 3 恐るべし
He did that homework in half an hour. 恐るべし!
(He did that homework in half an hour. He's amazing (awesome) !)
 
Example 4 恐るべし
He runs 5 miles everyday. 恐るべし。
(He runs 5 miles everyday. He is an incredible person!)