ELP is able to print a proper PCL print job in the reverse page order by using the ini-file key REVERSE_PRINTING. Means the printout starts with the last page and ends with the first page.

Once REVERSE_PRINTING=ON is set, ELP also searches for the key Reverse_Printing_Duplex. This key is mainly used, if the printing order for duplex needs to be face up. This means, that the front and back page are not switched. Maybe use 2 rules, determine if the actual job is simplex or duplex and use the command correct setting.
as a result a 4 page document is printed 3.4,1,2. A 5 page document: 5 (eject paper) 3,4,1,2.

Notes: