Since version 0.9 FreeModbus supports an lwIP  port. The port is pretty generic and the FreeModbus port should be portable to another lwIP platform by simply recompiling it.
In addition this port includes a complete FreeRTOS port for the Motorola Coldfire targets and an ethernet driver for the MCF523x FEC controllers. The M5235x platform is a very powerful targets and features a Freescale MCF5235 processor with 2Mb Flash and 16Mb SDRAM. The development toolchain used is the GNU C compiler 3.4.6 with the Insight debuggers . The files for this port can be found in demo/MCF5235TCP. The figure below shows a frame captured with the Ethereal network analyzer.