You make a good suggestion about the private note field, as I hadn't really considered that for tracking gas on the trip.
In the past, I used a trip entry as follows:
start date: 04/01/08
start odo: 100,000
end date: 04/14/08
end odo: 115,000
note: Trip to Florida for vacation. Then to Texas for visiting relatives. I-95 south, bla, bla bla. Car ran well; no major issues.
In this way the program would calculate the entire mileage for the trip (but not MPG or anything like that). It was really just a way to enter a note about a given time and mileage range. The only issue I see with using the private-note in the fillup log is that sometimes my trips don't always begin and end with a fillup. For me to calculate MPG during a trip, I just look at the fillups encompassing that time for a general estimate.
I also would use a trip-like entry (without the end date/odo) to record significant items like:
date: 4/1/08
odo: 100,000
note: check engine light came on, code reader indicated misfire in cylinder 1.
Right now I accomplish that with a note (service type Other) in the Maintenance Log. This way I can keep track of small problems and when they first started happening. There is nothing more frustrating than trying to figure out just when that rattle or shimmy started first appearing; by the time you consider it serious, you've forgotten when it started exactly.
Hope that helps. Perhaps both things are really the same, just with an optional end date/odo signifying a "trip" or extended time "note".