Not very familiar with the Isuzu trans., but might think about fabricating a "J" shaped dipstick tube (with a fitting nut that threads into the case drain). So you'd remove the existing drain plug, and screw in the new J-tube (with the open end of the tube sticking more-or-less straight up.
Cork the open end with a dipstick, mark the stick at the proper level (might require some experimentation), and there you have it.
Of course, I don't know how all the details would work out (or not). But the beauty of using the existing drain hole is that, it won't weaken the case, nor introduce chips into the trans., and if it leaks/ takes time to work out the details/ turns out to be a bad idea/ etc., you can always revert to using the plug, and still drive.
As far as using the dipstick from a 4-speed goes, nothing ever works out that nicely, for me, anyway 
