Yes this is an undocumented feature of Mach3.
There is a way around it though:-
You need to add some lines to the end of your M6Start.m1s macro.
This will read the tool data and place it in #1234 or any other # var of your choice.
D = GetToolParam(tool,1)
WD= Gettoolparam(tool,3)
SetVar(1234,D+WD)
Then in your gcode call the tool as normal but the G41/42 line will need to use the P command and the #1234 variable.
%
G21 G40 G00 G49
N1 (TOOL 2)
T2 M6
G54 G00 G90 G43 X-10. Y10. Z25. S2000 M3
Z1.
G01 Z-1. F250.
G42 X0 Y0 P#1234
Y-25.
G40 X-10. Y-25.
G00 Z25.
M30
%