Is it possible to identify a Serpent encryption key in memory? It has been shown that AES keys that are in use can be identified in memory. This identification relies on discovering expected round keys in memory that are contained within the key schedule. 

My question is: can we, by extension, assume that the same technique, at least theoretically, can be used to extract Serpent and Twofish keys from memory since they both have a key schedule as well??

(See also Extracting RC4 and 3DES keys from system memory.)
Yes, Serpent also does a deterministic well-known key expansion. So it should be possible to identify the sub keys in memory. You will want to know what implementation you are looking for since there is more than one sensible order for the key material to be in.