I wouldn't even bother with a charging circuit. Instead, get two 9V NiMH batteries, determine how long they last, then swap one with the other charged battery within that interval... charged externally, assuming the memory is retained as long as the unit has mains power, not completely dependent on the 9V for memory in that powered state. If it is dependent, just solder on a 2nd 9V connector then connect the new battery before removing the old - but don't dilly dally, the freshly charged battery will be discharging into the drained one, so you could use a switch or isolation diodes but really you just need to promptly disconnect the drained battery... 9V (PP3 form factor) can't produce a lot of current so that won't be a problem like it would with a higher capacity battery.
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