Hi,
If you need the users to approve something, this is a task not a mail (mail been only informative)
Tasks subjects are not limited to 50 chars, you can add links to decision tasks by attaching objects and as you said yourself in HTML tags not recognized bold chars will work in the SAP business workplace.
The second option I mentioned was to use a task which method calls a screen (to be more exact - calls a function module or a program which calls a dialog screen), and then you have all the options since this is a regular ABAP screen.
P.S
The extended notifications which inform users of waiting tasks is using a BSP application, in it html tags work very well.
Ronen