I guess as more of us age into this situation this question continues to be relevant.
The most useful "official" reference I could find -- one that clearly shows the different MAGI calculations for different uses -- is the Congressional Research Service's Dec. 2018 publication The Use of Modified Adjusted Gross Income (MAGI) in Federal Health Programs. It has a very handy Table 1 that lists 5 different federal uses of MAGI and what goes into each calculation.
With respect to the original question: when determining which bracket you fall into when determining IRMAA adjustments to premiums for Medicare Part B and Part D, untaxed social security income is NOT included in MAGI .
