I checked Rev 12 of my IFD540 Installation Manual and there are no connector pin assignments for a TOGA switch input and no discussion of such a capability. The Avidyne IFD540 Forum has one reference to a TOGA function and it basically says the 540 does not support it and recommends using the FPL L4 LSK to switch Enable Missed or set Auto Enable Missed Approach on the User Option page.
"Alternatively, the system can be configured to automatically enable the published missed approach for an active procedure by selecting the Auto Enable Missed selection in the User Options page. When this is selected, the system will automatically sequence to the published missed approach when crossing the MAP." It appears you have to use the L4 LSK to accomplish what you want to do before the MAP.
I gather the capabilities of whichever autopilot you have integrated with the 540 also has a lot to do with how the steering works in different modes.
John Salak
RV-12 N896HS