Thanks for the question. This is possible with a couple of simple expressions. As you've correctly identified, both scale and adaptive mode scale only on the X axis, so it is necessary to create our own scale mode that works on 2 or more axis. To do this you can use the variable XSectionLength
to find the size of the current spline segment. Wire this to a new Transform Node's X Fixed Size input. You've now pretty much created your own scale mode.
To resize the other two axis we can work out the scale factor between the original size of the segment and the length of the spline section. To do this you would need a sum something like - (XSectionLength/Input1)*100
where input one is the X Size attribute of the segment. This can then be wired to the Fixed Scale input of either of the remaining two axis.
I've attached a sample file with this setup. I hope that helps, please let me know if you have any further questions.