I don't have one yet but If you search the internet there are plenty of flexible dipsticks out there that have 1/4" NPT, especially in marine use and all you would need is a 1/4" to 1/2" adapter to make everything work. Here's one for an example.
http://www.cpperformance.com/p-13716-14-npt-flexible-dipstick-kit.aspx