Yeah, that's definitely on the soon-ish todo list. We'd like to allow open ended notes that aren't just fuel-ups. We were just talking today about allowing drivers to optionally also put in the costs of such things, so you can get a more accurate picture of how much it costs to drive your car each month, each year, etc.
We've also considered throwing up reminders every 3, 5, and/or 10 thousand miles that your car/bike might be up for some maintenance work.
Personally, I'd like to also add "business miles" as a non-fuel up note for my car's history, so that I could keep track of how much I've driven for work that I'll claim on my taxes the next year as a business expense (we'd definitely add the option to show all business miles from a single year so you could total it up at the end).
I'd say give us another week or two to get this going, it's definitely something we'd like to have on the site.