It would probably be more effective to write your own HTML that doesn't use the navbar widget and you can use whatever markup you want.
You didn't really state how many items you actually want to use in your navbar or what type of devices that you're targeting. However, you might want to think carefully about what you're trying to accomplish by adding more than five buttons into a navbar. This might work okay for larger tablets but may not yield the results you want on smaller phone form factors. If you really need to have this many options you might want to consider a different design strategy such as having a drop-down menu with a list of available navigation options.
If you consider the advice from the Apple Human Interface Guidelines, phone form factors don't display more than five items in a tab bar type control. This is based on the idea that you want to provide elements that are easy for people to actually be able to use.
Maintain a hit target area of at least 44 x 44 points for each toolbar item. If you crowd toolbar items too closely together, people have difficulty tapping the one they want.
The jQuery Mobile documentation matches up pretty closely to this advice, though admittedly it doesn't handle differences between phone and tablet form factors. Just because you could fit more buttons inline together doesn't necessarily mean it's the best thing to do, it depends on your use case.
Personally, when I'm building a mobile application one of my major goals is to make the application easy to use as a primary objective. Many people use mobile applications for some specific purpose to increase their productivity and aren't necessarily interested in having the kitchen sink on their device.
Not saying you're doing that here (hard to tell with the details that you provided) just something to think about with your design based on my own experience. What I did for my own application is just added a 'More' button as the
last button on the navbar and then I 'page' through the additional buttons
by dynamically modifying the icon, text, and URL for the navbar buttons. So, the buttons are still large enough to make them easy to use but it's fairly obvious that there are additional options available via the 'More' button. You could also use something like a select menu or popup to display additional options when selecting something like a 'More' button as well.