It depends on the cracks - are they just the surface or the fibres within have started to seperate.
All I can suggest is to try something - If you're thinking about sending the saddle to a saddler anyway then you don't have much to lose if it doesn't work.
You will need:
Bees wax ( soft wax if you can )
Cow horn ( the more worn the better )
heat source ( small blowlamp is good )
metal tablespoon ( with a wood handle if you can get one )
Cloth - fairly stiff ( I use the same sort as you would find in a resturant rest room that you use to dry your hands with - the towling on a roll in a machine )
Without saying -clean saddle , use glycerine soap, and apply conditioner, really rub the conditioner in with you hands, this should soffen things up a little.
Warm the beeswax ( don't melt it ) and rub it over the cracks ( use the same small motions you would as if you were colouring in a picture with crayons ) some of the wax will come off into the cracks.
Heat the spoon ( be very careful not to get it hot, it should feel hot to the touch but you should still be able to hold it ) , rub it over the beeswax , this should spread it thin and smooth things over.
use the horn - again rub it over the waxed areas this will smooth things further.
Lastly put some conditioner on the cloth and polish things over - best to do the whole area at this point .
Don't rub anything fast, use slow movements so you can keep an eye on what 's going on , especially with the spoon and horn.
If you want to do a really good job a little more prep is required, melt the wax and add in a small amount (about 10 % conditioner 90% beeswax) of the conditioner ( hopefully a grease ) . when cooled this will make a better substance to start with - but pure beeswax is OK.
You should not need any dyes or colourants.
be very careful not to heat things up too much - but you will need to heat them up - warm rather than hot
As with everything try not to rush things - you may need to repeat the process 2 or 3 times.
Let me know how you get on.