If you look on the datasheet for the equation they use to work out the 'trickle charge current' for the back up source....you can see that there are 8 possible current levels. Four resistors with one diode, and four resisotrs with 2(!) diodes. The diodes simply drop the supply voltage before it goes to the resistor, reducing the voltage across said resistor, therefore, reducing the current.
It all depends on your battery source. Pick a coin cell, and get the datasheet for it. Check the recommended charging currents...as as long as you don't go over the limits, you should be fine. As for 'supercaps', I don't see any reason at all to limit the current.
Theres also the question of power usage. The battery is only used as back up, to keep the RTC going, and to store the ram. But still, if it spends almost all its time in power down mode (running off battery) then when you DO give it a full supply thus charging the battery, you might want to jack up the current, otherwise it will hardly get charged. For example, if it is used for a datalogger..and it only powers up for a few seconds a few times a day? thats not much time to charge the battery.
Without more information, I can't help much.
Blueteeth
Note, I didn't want to say anything earlier but if you use a 'non rechargable' lithium coin cell..like the CR2032...don't charge it
Just use a shottky doide to block current going into the battery
see:
https://www.acmesystems.it/?id=105
personally, if you can, I would use a supercap just for the coolness factor.