I would caution against reporting as zero as that result might be a little misleading.
Instead I would change the custom field be: EQ(Peak Type,"Missing")*-1*60002 which will report "not detected" for your s/n custom field. A result of -60002 is a database code unique to Empower that will be translated as "not detected".
Alternatively, you could use the same custom field I suggested above as a boolean peak. The 0 (false) translation could be USP s/n (fc), while the 1 (true) translation might be something like main peak not present:. Though to be honest I'm not entirey certain if you can use a function boolean custom field as both a function (fc) and text. Also, I believe the use as function type of a custom field operator was added in Empower 2 FR2.