Guys if there is one thing I know , proper torque is proper torque. I don't care if its a 6 inch torque wrench or a 40 inch torque wrench. 25 ft lbs or 40 inch lbs or 15 nm is the same on a properly calibrated torque wrench no matter what size it is. And you can take that to the bank.
The tool for this job (please look
at Waldos picture) extends the length at which the torque is applied. The resulting torque applied to the fastener system depends on the ratio of the torque wrench length to the length of the torque wrench+tool length.
Honda supplies both the torque they want applied to fastener and the torque that would display on the torque wrench (if 20" long) to achieve that torque on the fastener.
I believe they suggest the bar type to prevent the reduction in 'applied to fastener' vs 'displayed on wrench' that would occur if the wrench had a ratcheting head introduced and angle between wrench and tool, reducing the overall length of the torque arm. The effect of the angle was shown in the diagram that Gator had posted.
I agree, any length can be used but it will change the ratio between wrench reading and applied reading. As long as the torque arm length difference (due to the length of the arm on Honda's tool) is accounted for any torque wrench could be used. (but Honda only did the calc for a 20") .
If Honda's special tool fastened on the torque wrench directly under the square drive, like most other sockets, instead of using the extension arm, there would be no calculation's to do. (and this thread would not exist)
EDIT: They built the tool this way because during the tightening sequence it is necessary for a socket to be used at the same time as the 'special tool'.
(pic on 14-28 of the same version of service manual)