Hi Jim, it sounds to me like the instant messageing feature in 1.5 should work: students message you with their code and you reply, messaging isn't threaded (that would be a nice feature for a CS class to add), but it does keep a record of a conversation, with dates and times, etc.
The forum with a separate group for each student would also work and keep threads, but it may take a bit to set up if you have large class.