Thanks. Well... I would need to setup WinUAE to work with the UAE-Configuration tool first, obviously.
For those interested in disabling JIT or switching to normal speed from the workbench, it seems the Green Alien Guide, wb 3.1, has some insights about it :
http://guide.abime.net/home3.htm
I wasn't aware of such a deep and complete guide on how to use WinUAE to WHDload via AmigaOS. 'tis really an Amiga emulator fan dream come true !
I will surely report if I manage to make my way out among all that stuff.
This said, I wouldn't mind if you clarified things a little. Does the above explanation somehow supersedes whdload tooltype tweaking ?