Yes It is possible and the complete feasibility depends on the devices that you are going to use.
You have to use Rhodes + RhoElements for this application.
What kind of devices (manufactures and model name) you are planning to use and what OS?
I am planning to use Motorola MC3000 and Cipherlab mainely. The OS is Windows embedded CE 5.0 and also Windows mobile 6.0.
In order to call an external DLL you need to build your own native extension. There is information on how to do that here:
and a webinar you may find useful here: