Mounting to the dash is always possible, particularly if you have a "taller" vehicle. Just be sure you clear your hood (especially if you have some kind of a hood scoop, including "reversed" induction hoods) as well as windshield wipers, etc.
Such a mount could be something as simple as a couple of squares of Velcro or 3M DualLock, to something more permanent such as a custom-made, bent-metal "insert" into a trim gap, such as what's available for mounting various aftermarket navigation devices (i.e. the ProClip for TomToms).
Alternatively, you could use either the factory-provided visor mount or an aftermarket visor-mount device.
Pushing the issue a bit, there are also specialty-market interior rearview mirror "stalk" mounts, such as these: Custom Made Connecting Mount for Radar Detector on 350Z and other Cars
and, for example, for late-model Corvettes: Great Lakes Trophies & Engraving - Corvette Accessories
Similar to such a "push" on the legality, you could mount the detector to your headliner, via the use of either a pre-made clip or custom-made, bent-metal or polymer-plastic "insert" that slots into the void between the headliner and the windshield, such as is seen here: Radar Detector Jammer Forum - View Single Post - 9500i passenger side visor mounting options?
's setup, using a piece of plexiglass Radar Detector Jammer Forum - View Single Post - 9500i : How do you mount your RD on your windshield ?
's setup, using the $4 Escort Visor Clip, clipped directly to his