The most straight-forward solution is two weapons, one two-handed and the other one-handed, with scripts that enables one that in turn disables the other. I wrote a pair of scripts years ago for someone who was creating such a mod. A claymore could be wielded one-handed if the player's strength was great enough. Later evolutions forced the player to fight two-handed if his/her fatigue drops too low. I cannot remember for whom I created the scripts. Perhaps you could search the usual places to download the mod if it was released publicly. It is possible that I have a copy of the mod someplace, but I have not played Morrowind in six years so if it is wanted it might take me a while to locate. I could find the scripts sooner if you are interested.
Actually I was thinking about adding a Strength/longblade skill requirement to weild claymores/dai-katans one handed and a longblade req to wield one handed swords, two handed. I figured in order to fully utilize the benefits of wielding a sword with two hands you'd need a certain amount of skill to do it. I wouldn't know how to go about adding that kind of requirement to the script though. If you wouldn't mind helping with a bit of the scripting I'd be very grateful. I just finished finals so I'll have to find a way to entertain myself lol.
Actually i just tested out the script from the Akatosh sword on Chrysamere and it worked well. I went on a killing spree in vivec to test it out and it was pretty cool chopping ordinators up with the one handed version. However while doing so i was thinking that it would have been great to have been able to actively switch between one handed and two handed during combat, depending on the situation,