This might be sort of a weird/rare situation (more common to have gap years in regular engineering industry/research labs), but how would the following be perceived-
Engineering undergrad with good research experience (ie. several publications) from a domestic U.S. school works in quantitative finance (something like quantitative research at a proprietary trading firm/hedge fund- work would be very math/modeling heavy as opposed to "traditional corporate white collar") for 2-3 years before applying to phd programs in their given engineering field (non-quant finance). Contacts would be maintained with undergrad professors (all prospective letter-writers would be faculty the student did research with instead of eg. professors from random classes).
Would this experience be detrimental for PhD applications in this scenario?