AvPlan also plots SIGMET's as well, and has done so for a while.
The problems with notam's and determining applicability to flight (basically adding some form of standard referencing to them) is why there is an emerging standard around digital NOTAM which Eurocontrol have been working on for a while.
Unfortunately its taken a while, and will continue to take a while before we get any operational benefit from it.
Bevan..