Hello gents, i would like to weigh in on this one if i may.
The only "castable" paste that I am aware of (paste directly to hook that is,and i stand to be corrected) would be a bread and cheese paste.
I used to make this as a kid, for carp fishing,by removing the crust from white bread, preferably a doughy type,and grating cheese(double gloucester or red leicester) and combining the two together to form a dough.This was reasonably castable on the hook, however it is quite easy to make some very castable pastes (50+yds on the feeder or for general waggler work) with the aid of a paste spring or coil,which i also make my self in various size but which can be purchased from any good tackle shop.
You will need a spice or coffee grinder (the average food processor is not up to the job) and you can pick these up in Argos or on ebay cheap enough £20 or there abouts.
a set of kitchen scales and the following
1. Halibut pellets (i find micros bind best)
2. Soya flour (you can use plain flour if desired)
3. Demerara sugar (preferably the dark one)
4. Salt
5. Chilli flakes
6. A mixing bowl
7. A kettle
This recipe will produce a tennis sized ball of paste and you can alter the quantities to suit yourself.
First put the kettle on, i boil 1 cup of water for every ball i'm making (saves on the "leccy" don't you know)
Now to your ingredients, measure out 70 gms of pellets and grind to a fine powder,then place in your mixing bowl.Measure out 30 gms of soya flour,throw this in the bowl also.
Add a dessert spoon of sugar, a pinch of salt and a pinch of chilli flakes.
combine all your dry ingredients thoroughly. By this time your water will have boiled and cooled down just enough (it needs to have cooled between 5-10 degrees)
Add a small amount of water to the dry ingredients and begin to stir (wooden spoon is best initially and place the bowl on a towel to stop it spinning around while you're mixing).
slowly add a bit more water until the mix starts to come together, when this happens abandon the spoon and use your hands, being careful not to scald them.
The ball should come together like a ball of firm dough, leaving little or no residue in the bowl - place in a plastic bag (i use the small blue tesco freezer bag). Remove the excess air, over hand knot the bag and in the fridge with it.
This will keep for a couple of weeks like this without any probs.
Another good paste especially for tench is made the very same way but i substitute bloodworm pellets for the halibut.
Not only is this a castable paste(with the aid of a spring) but there's no reason why you shouldn't use it direct to hook while on the pole,it doesn't melt away as quick as proprietry brands do and isn't it nice to catch on something you have made yourself ?
Hope this was of some value, thanks for reading
Seán