Xastir isn't bad at all but I recently encountered something called YAAC (Yet Another APRS Client). On it's site it states the following:
YAAC can be used as a stand-alone APRS client, an APRS RF-Internet
gateway (I-Gate), or as a AX.25 digipeater. It supports 16 different
data reporting views as delivered, and can be extended with
user-written "plugins" to add more functionality.
I'm sure there's already plugins available and with the AX.25 "digipeater" you should be able to also send packets. Keeping in mind it should automate much of what Scott Earle pointed out earlier: "crafting AX.25 packets" would really be a pain.
If you'd like to download it for OS X, it's available on Source Forge and the original website has source code available if you'd like to manually compile the software.
Note that "MacAPRS" has been discontinued although that too was an AX.25 iGate packet manager. Hope I've helped!