The HLC docs on HAI's knowledge base detail which links are free to use for any purpose.
Primarily the higher numbered ones. I posted the links below.
I use links to coordinate actions across rooms. I make sure to program the Links in both the HLC and UPB Scene areas in PCAccess. I find the UPB Link names easier to remember than A, B, C, D etc. The software will automatically translate a UPB link to the correct HLC Scene.
Here's what I've done in a few cases.
I use one of the Room links, say "D" for instance. I don't necessarily set the room links up in the 80, 60, 40, 20 format.
I may only use two of the links for the two primary scenes I use in that room. So I use the others for other functions.
If the switches them selves are not programmed to respond to the links, nothing will happen.
So in a room I may only have a portion of the switches respond to a given "room" link.
The Omni still updates the status for all switches listed in that room.
So in your case, let's say we used link D in the Mudroom.
Program the Mudroom light to react to link D (at whichever level you like), leave the other switches in that room off the link, meaning do not program them to respond to it. Then add that link to the Sink Light, so it will respond to it.
When you send Mudroom Link D ON, both the mudroom and Sink lights will come on simultaneously. Then the Omni will automatically send the status requests for the Mudroom. If you want to update status of the Sink light you can add a status command after the link command in the code for just that switch.
Here's something else I do with rooms. This may illustrate better what I am getting at.
I have all of our spare bedrooms as one "Room", but each link in that room controls only one room, but the Omni updates the status for all the bedrooms anytime it sees any link in the virtual room. The overhead lights are all CFL, so I don;t need dimming steps. I group similar items together and use the ON/OFF links to trigger and maintain status. I do the same thing with exhaust fans, exterior lights, etc.
Room ON (Bedroom 2, 3, 4 ON)
Room OFF (Bedroom 2, 3, 4 OFF)
Room LINK A (Bedroom 2 ON)
Room LINK B (Bedroom 3 ON)
Room LINK C (Bedroom 4 ON)
Room LINK D (Bedroom 2 & 3 ON)
So 99% of the time I'm using the local staus transmit option to update the Omni as we enter and exit the rooms and use the wall switch. When I use links I primarily use Links A, B and C. But the Omni checks status for all the bedrooms any time it sees any of the 6 links.
To turn off the bedroom I use Room Link A OFF (Bedroom2), the HLC convention is to use link only in the activate or ON mode.
But if you send a link OFF and it is one of the 6 links for the room, the Omni still updates the status of the room.
You could also do this.
Make the sink light respond to the Link D in the Mudroom.
That way it will turn on with the mudroom light.
In the code have it turn on the mudroom AND turn on the sink light.
The sink light AND the mudroom will go on with the initial link simultaneously.
The Omni will check the status of the mudroom at this time.
The Omni will issue the Sink On link.
The link will go out over the UPB net, but you won't notice because the light will already be on, but this second link WILL trigger the Omni to do a status update (assuming it is one of the room links the sink light is assigned.
There are several ways to use the HLC rules to get the effect you are looking for. It takes some creative problem solving.
In essence the only hard and fast rule is that the six links assigned to each room are hard coded to that scheme.
But you can make them do almost anything you want.
I don't use the Omni ALL ON/ALL OFF function.
I unchecked all of the HLC units from that option because I don't like the slow response.
Instead I use an ALL LIGHTS ON link to turn the lights on simultaneously, and told the Omni to use the link when an ALL ON or OFF command is issued. IIRC it still checks the status of all the HLC units because the ALL ON function was used, even though it din;t go through room by room and turn things on. (need to verify this when I get home).