Most importantly, where will you submit the bugs for your bug tracker before it's finished?
But seriously. The tools already exist, there's no need to reinvent the wheel. Modifying tracking tools to add certain specific features is one thing (I've modified Trac before)... rewriting one is just silly.
The most important thing you can point out is that if all they want to do is add a couple of specialized reports, it doesn't require a ground-up solution. And besides, the LAST place "your homebrew solution" matters is for internal tools. Who cares what you're using internally if it's getting the job done as you need it?
