Of course you can implement mission-critical software (whatever that is in your case) using Python. At the end of the day the success of your application is going to weigh more on its capabilities than whether it is written in Python. Some all .NET companies will even bring in Python applications provided that there is a way to talk to the system from .NET.
I would not market your application as being a Python application. This is going to cause you trouble down the road because you will run into roadblocks. This often happens when you satisfy a business customer and he speaks to their IT guy who says "whoa we can't support that" without a full analysis of the cost/benefit to the business. This is the place that references to use of Python in mission-critical systems will arise. Try to avoid this area.
With Python you can always target the popular platforms if you build your application under certain constraints. IronPython runs on .NET and Jython runs on Java. Being able to fire back with info on how to run your application on these platforms might be helpful.
