Ant,
The conventional cylinder hone route should be OK I would have thought, if your mold has a constant draft angle, and you can get (or adapt) one with suitable length arms.
The
hones are pivotted and will take up the angle (within reason) of the taper, In operation, the drive shaft needs to be on the axis of the cylinder; the stones are then kept in contact by the centrifugal force as the hone is spun up.
It won't do well though if you try to operate a smallbore hone set off-centre in a large cylinder - the independently-pivotted arms will just flop about, giving effectively interrupted cuts - but, if necessary, making a longer set of matched arms shouldn't be hard.
The hone assembly does need to be spun-up to get the hones out to their operating radius - unfortunately it won't for example, work by having the hone mounted in a tool holder or in the tailstock, and then spinning the work still mounted in the lathe. Best would probably be to mount the hone in a mill or drill press with the work centred underneath and raise/lower the quill to work the hones.
If the diameter of your mold is too great to use a set of hones, another alternative would be some form of toolpost grinder (search on here); in which case you'll need to leave the grinder set at the same angle you had on the compound when you turned the original taper. If you are going with your reciprocating sander, as Joe/Ralph have highlighted, you'll need a smaller drum so it is only in contact with one wall at a time, and the action shaft will have similarly have to be set at the same taper angle.
Dave