Like you said, building the habit is the hardest part, but this type of introspection really has it's benefits once you get going. I've been keeping a diary for years as just a word file in a truecrypt volume. Most of the apps I found in this market were trying to be more of a blogging/sharing platform rather than a private journal.
Looks interesting, I've been thinking about building a journal application just for myself for a while (none of the ones I looked at ever did what I wanted and since I'm a programmer that isn't an excuse I can really live with).
I think for the engagement thing I'll have mine send me an SMS (Nexmo makes this very easy), Emails I silence outside of work but SMS's I always check (as that's how I know servers have gone down).
I have a super-simple REST API setup for the site (documentation isn't online yet), so if you can roll out your own service how you like and use my endpoints for the storage/synchronized access.