Thanks for the excellent idea. You can customize your form yourself by going to tools>forms>Design forms, and then choose the form you want to modify. Then, either save the form to replace the old one, or save as a new form. If you opt the later, you must go to the task folder and change its property to use the form for posting in "when posting to this folder, use: [drop down list]". It should not be a hard one to customize, for what you want is only to change the default field.
However, I have a little paranoid to keep things as what they are meant to be. Undated would rather stay as undated, instead of 10 years later. I hesitate to tune & sync the whole system (my PDA, Outlook, and most importantly my intuition) to a drastic change of convention just for the sake of sorting. It's only personal taste, and this is an excellent idea of yours.
Apart from my wish for MS to make outlook more sensible, I hope there's still other options