You already have 25% chance of getting +2 XP for each you logged in, so to an extent, it already exists.

... Have the bot randomly message people with short but warm comments...

I would be against, unless it's an option you can switch off:

<input type="checkbox">Don't listen to the "happy" monk
[download]

But seriously, you would have to be very careful in your wording, because interpretation can be very culturally determined.

Anecdote alert
I used to know an (American) interactive teaching program that would ask questions as 4-choice multiple choice questions. No matter how many tries you would need to get the right answer, once you got the right answer, the feedback would be "Great!". Apparently this was considered comforting and reassuring in the States. Over here in Europe, that feedback was considered to be very cynical, especially if presented after 4 tries.

So what might seem reassuring to you, might be construed as very cynical and downputting by others.

Liz