As per the best of my knowledge, scFv is a fusion protein and it is highly convenient to express the antigen-binding domain as similarly as a single peptide. So, I guess you can use HRP monoclonal conjugate secondary antibody for the detection.
I always do this with an anti-tag mAb, because most scFvs are fused to a tag for detection or purification (His, myc, FLAF tag or any other). But I know people producing thir own anti-scFv antisera. Perhaps some commercial anti-Fab sera are crossreactive to several variable regions and thus useful, but I haven't tried.