There isn't a really elegant way to make it happen, but for a given userId you could successfully return your the most up-to-date event create a count query with a group_by on [surveyScore, keen.timestamp] and an order_by on the keen.timestamp property. You will want to set limit=1 to select only the most recent surveyScore.
If you'd like to use an extraction, the most straight forward way would be to run an extraction with property_names set to ["userId","keen.timestamp","surveyScore"]. Once you receive the results you can then do some client-side post processing. This is probably the best way if you want to take a look at all of your userIds.
If you're interested in a given userId and want to use an extraction, you can run an extraction with a filter on the userId eq X, define the optional parameter latest set to latest=1. The latest property is an integer containing the number of most recent events to extract. Note: The use of latest will call upon the keen.created_at timestamp instead of keen.timestamp (https://keen.io/docs/api/#the-keen-object).