This small scripting change will change all seige engines (except small bombards) to have fire/ cow carcasses again, but leaves archers as they were. All you have to do is replace descr_projectile with the one here (see bottom), and then edit descr_unit as shown:
Scroll down to trebuchet and find this line: (this is to allow carcass shot)
stat_pri 6, 3, no, 0, 0, melee, artillery_mechanical, piercing, knife, 25, 1.8
;stat_pri_ex 0, 0, 0
stat_pri_attr no
stat_sec 55, 3, trebuchet, 285, 30, siege_missile, artillery_mechanical, blunt, none, 25, 1
;stat_sec_ex 0, 0, 0
stat_sec_attr ap, bp, area, launching
Then add this underneath:
stat_ter 12, 3, cow_carcass, 215, 3, siege_missile, artillery_mechanical, blunt, none, 25, 1
;stat_ter_ex 0, 0, 0
stat_ter_attr ap, bp, launching,
so it looks like this:
stat_pri 6, 3, no, 0, 0, melee, artillery_mechanical, piercing, knife, 25, 1.8
;stat_pri_ex 0, 0, 0
stat_pri_attr no
stat_sec 55, 3, trebuchet, 285, 30, siege_missile, artillery_mechanical, blunt, none, 25, 1
;stat_sec_ex 0, 0, 0
stat_sec_attr ap, bp, area, launching
stat_ter 12, 3, cow_carcass, 215, 3, siege_missile, artillery_mechanical, blunt, none, 25, 1
;stat_ter_ex 0, 0, 0
stat_ter_attr ap, bp, launching,
Have fun setting them alight!- or being set alight