Eureka! From page 189 of Michael J.F. Bowyer's bombing colors: "To hasten delivery the word SNAKE was applied aft of the serials on some Liberators passing through the Middle East to the Far East, but this seems to have been applied infrequently." Page 188 contains a drawn profile of Liberator KP136 wearing the SNAKE marking and panel no.91 on page 168 is a photograph of this same aircraft.
Although this discovery does not answer all possible questions about SNAKE, it does satisfy me that I have finally been able to lay my hands on the reference that I remembered from many years ago a