The key for this type of setup is to make sure that they understand your job is like that of a security guard. You're there for when all your carefully coded alarms start going off. You're not there to help shovel the sidewalk because then you might miss your alarms.
As long as that is properly understood you can get a contract that is permissive enough to allow you to work on something else at work when nothing is burning. Even if you have to offer a reduced salary, it's worth it.
Another option is to go the work on OSS route, especially on tools you use for work. After a couple years of doing semi-fulltime OS work you'll be commanding double the salary.
The key to this, and I meant to edit this into my comment, is to be super-responsive to any requests. Always be willing to drop whatever you're doing at a second's notice and help whoever it is that thinks you might be able to help them out. The word gets out and everyone in the company just loves you.
As long as that is properly understood you can get a contract that is permissive enough to allow you to work on something else at work when nothing is burning. Even if you have to offer a reduced salary, it's worth it.
Another option is to go the work on OSS route, especially on tools you use for work. After a couple years of doing semi-fulltime OS work you'll be commanding double the salary.