I like it.
We've considered something similar if this baby is a boy — we both like Sam, but are lukewarm on Samuel/Samson, etc. But two of our family names are Silas and Americo, so we've considered Silas Americo nn Sam.
You can also do Mary Agnes as a double first name and a G middle name. My mom's initials are MEG and she's always gone by Meg.