\documentclass{article}
\usepackage{biblatex,xpatch}
\ExplSyntaxOn
\NewDocumentCommand{\xpatchfieldformat} { O{*} m }
{ \xpatch_main:Nc \patchcmd { abx@ffd@ \tl_to_str:n {#1} @ \tl_to_str:n {#2} } }
\NewDocumentCommand{\xpretofieldformat} { O{*} m }
{ \xpatch_main:Nc \pretocmd { abx@ffd@ \tl_to_str:n {#1} @ \tl_to_str:n {#2} } }
\NewDocumentCommand{\xapptofieldformat} { O{*} m }
{ \xpatch_main:Nc \apptocmd { abx@ffd@ \tl_to_str:n {#1} @ \tl_to_str:n {#2} } }
\NewDocumentCommand{\xshowfieldformat} { O{*} m }