xAP for the PIC uses Microchip's free assembler. The assembler offers an excellent simulator to assist with debugging, and in-circuit emulators are available for many PIC devices, although these are generally costly.
A PIC assembler library is available which supports table driven parsing of xAP messages - the code is very compact, occupying around 500 bytes.
There are a number of Basic compilers around for the PIC, and although I don't have first hand experience of these, I believe that integrating the assembler library with these shouldn't be too difficult. If you have success, do let me know.
PIC's typically interface to the network using a serial interface, so if you intend to run xAP applications on your ethernet network as well as on PIC devices, you'll need to set up a xAP serial bridge to pass messages between the two networks.