Yes, if I understand your question correctly, please clairify if I am not understanding what your asking.
given your example, your saying you want to turn on some OUTPUTs that would handle drive enables, across the non-plugin serial modbus correct? And you want to control that via VB? You can do it (control the Plugin-Serial-Modbus), by VB indirectly.....
Lets say you want to turn on OUTPUT1, or, even Enable1 and have it go across the pluginenabled modbus.
in VB you set, some trigger, or get some trigger, i.e. GetUserLED, or SetUserLED for example, as your input in Brians, send that result to
a termination of outputxx, and set another rung, that looks at outputxx and sends it to a Plugin enabled modbus output.
scott