Unfortunately, product-specific MEP embodied carbon data is currently lacking. As far as I can tell, there isn’t enough data to develop a baseline against a proposed design.
However, you could use recycled content of the steel as the variable between baseline and proposed and approximate the MEP equipment weight as steel sheets.
Then, you could use generic steel sheet embodied carbon data:
(% recycled content, kg CO2e/kg of steel sheet) [Source: OneClickLCA generic data]
You would determine the overall weight of the MEP systems and recycled content of baseline and proposed materials to get a very rough approximation of embodied carbon differences.
Note that it may be difficult to get the recycled content percentages from the manufacturers.