I suspect the answer to this is "no" (I couldn't find any obvious match on the driver capability list), but is there a capability that could be used to return positive/negative magnitude values from a continuously turnable dial?
Not specifically. The purpose of Capabilities is not to limit development. It is instead to provide common definitions system-wide for the commands, attributes, and data for use throughout the system. But they does not limit the developer from creating a driver that requests/reads and then reports as attributes the absolute value, the latest change, and the direction of change (increase/decrease).
I would use the capability "Sensor" to announce to applications (dashboards, rules, etc) that the device is a sensor. Then, create the method(s) to communicate with the rotator. Finally the methods to compare to current value and if different update the attributes. Final would be the internal use of the data (if any).
Thanks for the input; I do have a (just about) working implementation using an attribute name of my choosing, but before taking things any further thought it would be best to check whether there was an existing framework it should be slotted into. Partly for tidiness's sake, partly because using an "official" Capability might make it easier to use via standard device types in RM.
From what you've said the Sensor option does look hopeful, so I'll give that a go and see where that takes things.
It is usually good for any custom attributes you might create, to not overload existing attribute names if you can Avoid it.
It can help when reading applications / rules/ pistons that use the device to avoid confusion.