I looked at the Kobo driver code. There is a way to get partially there using a plugboard.
The driver sets the series name and index to whatever the plugboard produces. You can't have two series numbers so you can't have a subseries index, but it does accept floating point indices so you can encode the subseries into that.
My thought: for a book with series X[10] subseries S[4]. Use the plugboard to set the series to 'X - S', and to set the series index to 10.04 (zero-padding the subseries index). This will sort on the kobo first by the series-subseries name, then by the index.
For example:
- Series X[4]
- Series X[8]
- Series X[5]
- Series X[5] Subseries Q[1]
- Series X[5] Subseries Q[11]
- Series A[20] Subseries B[4]
- Series A[20] Subseries B[21]
- Series A[4]
- Series A[21]
we would get on the Kobo (sorted)
Code:
A [4]
A [21]
A - B [20.04]
A - B [20.21]
X [4]
X [5]
X [8]
X - Q [5.01]
X - Q [5.11]