In the Work Schedule Rule (V_T508A), we used an unused field (M2STD) to hold the %. Note that all other "Working Time" fields are filles as if the employee was at 100%.
In table PA0007, our ABAP resource created 2 additional fields, one to hold the % comming from V_T598A, and one to hold the number of hours per Pay Period.
When creating the employees' IT0007, the "Special" WSR is entered without any indication that the employee is "Part-time" other than the description of the WSR.
Upon saving IT0007, the system first fills the 2 additional fields based on the Work Schedule Rule. The % is taken from field V_T508A-M2STD, while the number of hours per period is the multiplication of the Daily Working Hours (V_T508A-TGSTD) times the number of work days in the Payroll Period times the % from the other Custom Field.
Through the ABAP code, an IT0008 is then created with the same Start Date and with the Data from the 2 Custom Fields from IT0007.
For each ESG Grouping, we were limited to 8 "Special" Daily Work Schedules as we took advantage of the unused Daily Work Schedule Class to create Absence Deduction Rules that allowed one day of Absence to impact the Quota based on the Daily Work Schedule.
Lots of work, lots of limitations, but the result was something that suited our needs.