This is not as much a question as it is a statement.
The modbusgateway binary in "/usr/sbin" is not feature-complete from the perspective of the Modbus protocol. It lacks support for reading/writing coils (and discrete input) along with support for reading input registers. Functions 1, 2, 4, 5 and 15.
So, I took it upon myself this weekend to create a working alternative.
This software aims to be a drop-in replacement for the current binary located in package/base-files/files/usr/sbin/modbusgateway in the SDK for the RUT9XX series.
A pre-compiled binary for the `MIPS32 rel2` target can be found at https://github.com/ganehag/open-modbusgateway/releases/download/v0.1/modbusgateway if anyone wants to try it out.
Fixes and improvements are welcomed.