I use HGS to connect AC500 to HPC800. The analog values can not be read on the composer
%MW0.4 should also be address 4 in the HGS modbus command record to read the word data from the PLC. The mapped address is to suit your application. As the digital signal worked when configured as a modbus coil try using the read holding register command, FC 3. You will also need a HGS mapping table entry to define the analogue data type and make the data available to the function code environment.
A snip form the Automation Builder 2.3 help