Is it possible to set up reminders for custom service/maintenance tasks?
Yes, you can add service reminders for more service types by adding "custom service types" for your account. Click the "Service Types" link under the Account drop-down link on the top navigation bar, then you can add/edit your custom service types from there.
Your custom service types will then be available as options on both the Service Reminder form, as well as the Service Entry form. You'll see them in addition to all the built-in options.