That would be great if there was a ability to store the IOT and index for ST geometry data in separate tablespaces through the use of DBTUNE keywords.
To this day A_INDEX_SHAPE parameter is not valid parameter for keyword with ST_GEOMETRY storage (it is also nowhere mentioned in documentation). And to this day spatial indexes of delta tables are stored with spatial indexes of base tables.
I think it would be wise to make A_INDEX_SHAPE valid parameter and to store spatial index objects in tablespace described in this parameter.