Apologies for missing your original post, @Sebastian_Therman!
More native support for lightweight timestamping is still ongoing. You can track progress at https://github.com/opendatakit/javarosa/pull/265.
In the mean time, I've realized that similar results can be achieved with a technique I described in Is it possible to set a default value for a decimal question using the calculate column? - #3 by LN The idea is to call once(now()) the first time the question you want to time gets a value. Here is a sample form.
As of Aggregate 1.5, it's also possible to gather and receive a log of enumerator activity as a CSV attachment. See https://github.com/opendatakit/docs/pull/649 for a first pass at documentation. Comments and suggestions appreciated!