I can't tell a lot about builds, but the only thing I can say is that horse archery skill doesn't affect anymore throwing on horseback/on foot.
I've been thinking with jarids and a cavalry sword (for hybrids, foot throwing in town maps):
level 30
15/18
HA :3
IF: 4
PS: 5
ATHL : 6
RID : 6
WM : 6
Throwing : 146
1h : 45
I wouldn't advice you to go for such a class if you aren't experienced with it on foot, because this build is the only one to provide some usefulness on foot, and some resistance at level 30 -> it becomes better to have a 18/18 thrower on high levels like 32/33. If you want to go pure horse throwing, remove PS , ATHL, and IF, make a 18/18 instead, or a 15/21 if you believe you can use the speed of the horse to give some damages. San gave you a good dedicated horse throwing build tho, but once you're on the ground, you're screwed, like with every dedicated cav classes. Make your choices between these builds, looms can only help you out, since it's hard to play, and unconventionnal.