I found some more answers at the website Jean recommended.
In brief,
" 6. Use of tethers for children over 40 pounds (Sept 03)
Confusion over Child Weight Limits for Tether Anchors Must be Cleared Up
The issue of maximum child weight limits for CRs using tether anchors as defined by some vehicle manufacturers has become a hot issue that pits children’s needs against vehicle manufacturers’ limitations (page 4). CPS technicians, advocates, and parents want and deserve to know the real margin of safety for children over 40 pounds.
There seem to have been different interpretations of the very unclear language in the discussion in the final FMVSS 225 rule regarding test requirements for tether and lower anchors. NHTSA needs to communicate to vehicle manufacturers the implications of its standard. Child restraint makers who took the initiative to produce CRs that usable with a harness and tether for children above 40 pounds need to stay on course until vehicle instructions catch up with them.
This issue should be resolved as quickly as possible to minimize the number of years for which product instructions will be ambiguous. It is very unfortunate that the benefits of tethers for higher-weight children who need them may be lost due to manufacturers’ overly-conservative estimates. While there will be no changing the anchors in existing vehicles or the text of current owner’s manuals, CPSTs and advocates need to know the true story so they can advise people appropriately.
Two closely-related petitions must also be dealt with by NHTSA. The agency should address how families needing to tether restraints for children over 48—65 pounds can obtain assistance to install stronger hardware appropriately, as suggested in the 1999 petition from SBS USA. The sanctioning of tether use above 40 pounds for combination seats with harnesses also begs for consideration."
More info:
http://www.saferidenews.com/html/LATCH_P49.htm
Car Manufacturer's stated maximum weights for LATCH and Tether anchor points:
http://www.saferidenews.com/pdfs/LATCHApp_B_pg105.pdf
It sounded like most of the car manufacturer's had not done actual testing to determine these maximums, however.