My comment would be the same as Wildherre's..yes, they will work but you won't get a lot of time from them. If they start out say at 1.3 volts per cell (typical, even though the nominal is 1.2 volts for NiMH) you will have a battery with 10.4 volts terminal voltage. If you view my schematic of the power supply section of a Soveeign that is posted on Wildherre's website, you will see that a +5V and a -5V supply is generated from the potential across the battery plus some overhead loss across a series regulator. So you basically start out with no battery voltage reserve and once the terminal voltage of the cells drops, you are no longer going to be working with a regulated supply, things start to get erratic and unpredictable. So, the extra 2.5 volts is critical to ensuring a well regulated supply.
I run both of my Sovereigns from a belt-mounted 2.2 AH SLA (sealed lead acid) pack that they sell at Ham Radio Outlet. I have added a curly-cable from an old cell phone automobile/cigarette lighter adapter from my battery compartment to the cigarette lighter receptacle on the battery pack (I stripped out the regulator guts in the plug and bypassed them). This gives me more battery time than I have ever been able to use, removes the battery weight from the control box, and allows easy connect/disconnect of the battery pack to the detector. Sovereigns work fine with SLA packs, and the shelf life of the battery is the best of any of the rechargeable technologies. A little bit heavier in mass per ampere hour but you don't even notice the difference when it is on your belt.