It should work: Just tried it with your code (but in a Word VBA macro) and
got the following:
Set field us-ids[0].sfpatitemflow[0].patItemsf[0].patnumbertxt[0] = 123
jso.GetField(us-ids[0].sfpatitemflow[0].patItemsf[0].patnumbertxt[0]) = 123
Set field us-ids[0].sfpatitemflow[0].patItemsf[0].patnumbertxt[1] = 50
jso.GetField(us-ids[0].sfpatitemflow[0].patItemsf[0].patnumbertxt[1]) = 50
Set field us-ids[0].sfpatitemflow[0].patItemsf[0].patnumbertxt[2] = 100
jso.GetField(us-ids[0].sfpatitemflow[0].patItemsf[0].patnumbertxt[2]) = 100
Set field us-ids[0].sfpatitemflow[0].patItemsf[0].patnumbertxt[3] = 200
jso.GetField(us-ids[0].sfpatitemflow[0].patItemsf[0].patnumbertxt[3]) = 200
