In Python the method SendDREF(s) fails when packing a list passing at the following location:

if hasattr(value, "__len__"):
    if len(value) > 255:
        raise ValueError("value must have less than 256 items.")
    fmt = "<B{0:d}sB{1:d}f".format(len(dref), len(value))

    # The next line will fail
    buffer += struct.pack(fmt.encode(), len(dref), dref.encode(), len(value), value)
else:
    fmt = "<B{0:d}sBf".format(len(dref))
    buffer += struct.pack(fmt.encode(), len(dref), dref.encode(), 1, value)

The failure depends on the size of the array being passed. For example if passing an array of size one the following failure is reported:
self._client.sendDREF("sim/multiplayer/controls/yoke_roll_ratio", [1])

buffer += struct.pack(fmt.encode(), len(dref), dref.encode(), len(value), value)
struct.error: required argument is not a float

If passing an array larger than one the error is different:
self._client.sendDREF("sim/multiplayer/controls/yoke_roll_ratio", [0, 1])

buffer += struct.pack(fmt.encode(), len(dref), dref.encode(), len(value), value)
struct.error: pack expected 5 items for packing (got 4)

For the DREF in question the docs state that I can accept an array of size 20:
yoke_roll_ratio | float[20] | 860+ | yes | [-1..1] | The deflection of the axis controlling roll.

For my purposes I did a quick and dirty fix that only works for lists up to size 2:

if len(value) == 1:
    buffer += struct.pack(fmt.encode(), len(dref), dref.encode(), 1, value[0])
elif len(value) == 2:
    buffer += struct.pack(fmt.encode(), len(dref), dref.encode(), 2, value[0], value[1])
else:
    raise "Array of size " + str(len(value)) + " Not implemented"

After putting this kludge in the DREFS in question adjust the simulator as they are supposed to.