Allowable reverse current into Alkaline battery
Self-discharge of a 9V alkaline with a 20% down shelf life of 5 years and 500mAh capacity is about 2uA, so I don't think that &lt;2uA is any kind of a possible problem. There is no net charging taking place. 

On the other hand, Schottky diodes can have very high leakage at high ambient temperature. The self discharge current will also go up with temperature but probably not as fast. So you may want to consider the characteristics at the maximum possible ambient temperature. 

So much for logic, let's see what, if anything, the manufacturers recommend. 

Eveready (Energizer) has the following blanket disclaimer in their manual (presumably to deal with abuse such as trying to actually recharge alkaline cells at high current, causing heat, evolution of gas, explosion etc.) : 


  Charging of Primary Batteries:
  Charging of primary batteries may cause explosion or leakage which may result in bodily injury. IF
  ENERGIZER/EVEREADY PRIMARY BATTERIES ARE SUBJECTED TO ANY FORM OF RECHARGING, ALL
  WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, ARE NULL AND VOID.


However when you go into their "Design and Safety Considerations", we finally get a real, and sensible, number (emphasis added): 


  Devices with alternate power supply options should be designed to assure the
  batteries are isolated from the alternate power supply circuit. Use of external switches,
  activated by the alternate power supply cord should be avoided. Commercially available "generic"
  replacement cords may not activate the protection switch. Consider the redundant diode
  protection against unintended battery charging. Maximum leakage current should never exceed
  35 micro amps.


Even allowing for variation between types and manufacturers, it would seem that &lt; 10uA is very, very safe. It's up to you to determine whether some sensible limit like that is respected.