Hi Debster,
Unfortunately, there are very few options for five point harnesses above 40#s. But there are three and I believe 2 are FAA approved (this is why you are having so much trouble finding one). Other than a seat with a higher limit for the 5 point harness, your only other option for above 40#s is a belt positioning booster. None of the belt positioning boosters will be FAA approved because boosters need shoulder belts in addition to lapbelts to function properly. No airplanes have shoulder belts, so there is no point in searching for an FAA approved belt positioning booster, it simply will not exist (sorry). All the Graco, Evenflo, Cosco, etc. combination (forward facing carseat that converts to a booster) carseats that are currently on the market are approved for use with the harness only until 40#s. After 40#s, the booster portion must be used. Many claim they can be used to 80#s. This can be true, if your child doesn't outgrow the harness by weight or height before the late 4's. Most 3 years olds are not mature enough for a belt positioning booster. In general, most 3 year olds wiggle a little too much (mine does!) and have a tendency to fall asleep. A harnessed seat is much safer (especially for a 3 year old), but it is ultimately your decision on what is best for your child. As far as the five point harnesses that are available for above 40#s, they are:
Britax Marathon -- harnesses to 65# I've read multiple posts (other boards) from people successfully installing in airplanes. This is a rather expensive seat (usually $200 plus), but has many safety features that make it easier to use.
Britax Husky (used to be Super Elite) -- harnesses to 80#, but very bulky and not FAA approved.
Nania Airway -- harnesses to 50#, then converts to belt positioning booster. I did find one spreadsheet that says this is FAA approved, but I do not have experience with this brand. This is better priced (around $100), but difficult to find except online. I have also heard that the harness is threaded differently than what we expect in the US (it is a European seat). It is possible to rethread the seat harness to make it look more like a US seat. If you want more info on that, either post back here or post at the car seat forum at parentsplace.com (multiple carseat techs post on that board and would be willing to help)
Please, please let me know if you have any further questions.
Take care,
Jennifer