There are several reasons why the method you describe will not work.
You cannot put a 1756-DHRIO module into a chassis along with a redundant ControlLogix controller. Only CPUs, Ethernet modules, ControlNet Modules, and System Redundancy Modules (SRMs) can reside in the controller chassis.
I cannot speak for Honeywell and the capability of your TDC3000 system (there is more than one "serial gateway" type device for TDC3000) but in my experience the SIO module can only perform "Local DF1" messaging. If a 1756-DHRIO is located outside the processor chassis, it cannot use a local message because the target of a Local Message must be a local slot.
In addition, if your DH+ network is principally used to interface with Honeywell TDC-3000, then it probably includes some 1770-KF2 or 1785-KE modules. In my experience, the 1756-DHRIO module can have some DH+ problems when only -KF2 or -KE modules are on the network with it. When PLC-5 or SLC-5/04 controllers are present, those problems do not occur.
My very strong recommendation for communicating with a redundant ControlLogix system from a TDC-3000 control system is to use a Prosoft MVI56 module in a remote chassis. The Modbus version (MVI56-MCMR especially) or the DF1 version (MVI56-DFCMR) can be used depending on which protocols are in place on the Honeywell side.