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 <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 < 10uA is very, very safe. It's up to you to determine whether some sensible limit like that is respected.
The problem with batteries is that all kinds of weird/nasty chemical reactions can happen when they're mistreated. Those reactions can lead to corrosive substances leaking out of the batteries. We've all seen batteries that have leaked and the mess they can cause.
Unless a manufacturer specifies how much reverse (actually charging!) current is allowed you're well advised not to charge the battery.
But why not simply use a battery that is designed to be charged. There are 9 V NiMh cells that can perform the function that you need. When using a rechargeable cell you can even add a resistor in parallel with the Schottky diode to increase the charging current to about 1 mA or so. That way the NiMh battery well be kept in a charged state.
If a NiMh 9V battery is too expensive for you (the cheap ones aren't of good quality, avoid those) then use a silicon diode (1N4148) instead of the Schottky diode as silicon diodes have much lower leakage currents. So much lower that the current cannot be of issue. You will just have to live with the extra voltage drop of the Silicon diode.
There are also "ideal diode" circuits and ready made modules with such circuits on them (these use MOSFETs to do the switching) but I am unsure if these will provide the very low leakage current that you would need.
Can't put safety and alkaline in the same sentence. Alkaline batteries leak, through the valve meant to stop explosion when they are about to leak.
For a good alkaline, 0.1uA is nothing. A good freshly depleted alkaline can even be recharged to about half of original capacity. But do you know if the alkaline is in good condition? I've had alkaline batteries from all the top manufacturers leak years before expiry date, sometimes in the sealed packaging, sometimes in gadgets that were then destroyed due to the leaked alkaline cell.
If the current draw from battery is low and the battery needs to be installed for an extended time (more than a month), use heavy duty zinc chloride. Nowadays they don't leak, cause minimal damage if they do leak, are nearly as good as alkaline in mAH/price. Buy fresh though, as they start deteriorating after manufacture.
Although it sounds bad, if price is a concern, for short term backup, one can consider lead acid batteries. The tiny 4V ones come at 250-300 mAH. They will tolerate a floating voltage forever. They may cost about the same price as an alkaline AA. Also, you can get them in any mAH you may require, and AH/price seems to be almost linear for all sizes.
NiMH does have some limitations. They are expensive, cannot be trickle charged, risk of polarity reversal when a bunch of them are in series.