Palm has a simulator that acts like the TX. You first have to register at PDN (Palm developer network) here:
https://pdnet.palm.com/wps/portal/pdnet/developers then download the TX simulator. Unfortunately, there is still no ARM emulator in this one.
If you need to use ARM, you're ARM code is going to have to be compiled as a .dll file (with the Palm OS compiler), and that .dll will go into the folder your simulator is in, and you will also get a prc file containing other resources like forums, alerts, and 68k code, and it will be installed into the simulator.
I don't know how to configure the Palm OS compiler to compile as a DLL, but if your app is 68k only you should be fine.