Yes, you can put anything you want in the messages. The only caution is using a lot of punctuation. I have to "clean" the input for some geeky reasons so keep it simple.
Also, your use of custom messages is a good one. I may modify the code a bit so that a custom message will count as an OK.
The reason for changing from every 4 hours to about every 4-6 hours is to make it clear that it is not a rigid time schedule. The point is periodic OK messages while on the water becuase they are positive affirmations rather than just automatic track ticks. Having the OK timed to some periodic function that you do throughout the day is a good way to comply with the guidelines.
