most of these accessory bars that i've seen are mainly made to hold bike computers and lights only as well as other tiny trinkets instead of an actual bag.

but if you have a typical cromo seat rails can't you just buy 2 carabiners and attach it to the loops on the bag? only problem i see if the side to side swaying but if it is just a small bag it shouldn't be too much of a problem. this would totally depend on the bag you are mounting on there.