Three-letter style
While I know the intention may be to have a custom style since not every style can be included, I think the more modern three-letter abbreviation style is common enough (perhaps the most common in the books I've read) to be included as a standard style. This is what I've been using:
\newcommand*{\brs@custom}{%
\brcustomabbrvname
\renewcommand*{\BRbooknumberstyle}[1]{##1}%
\renewcommand*{\BRepistlenumberstyle}[1]{##1}%
\renewcommand*{\BRbooktitlestyle}[1]{##1}%
\renewcommand*{\BRchapterstyle}[1]{##1}%
\renewcommand*{\BRversestyle}[1]{##1}%
\renewcommand*{\BRversesuffixstyle}[1]{##1}%
\renewcommand*{\BRbookof}{}%
\renewcommand*{\BRgospel}{}%
\renewcommand*{\BRepistletothe}{}%
\renewcommand*{\BRepistleto}{}%
\renewcommand*{\BRepistleof}{}%
\renewcommand*{\BRbkchsep}{~}
\renewcommand*{\BRchvsep}{:}%
\renewcommand*{\BRchsep}{; }%
\renewcommand*{\BRvrsep}{--}%
\renewcommand*{\BRvsep}{, }%
\renewcommand*{\BRperiod}{}%
}
And the three-letter abbreviations. Standards found here: https://www.bibleworks.com/bw9help/bwh42_Abbreviations.htm
%% define custom abbreviations.
\newcommand*{\brcustomabbrvname}{%
\def\br@Genesis{Gen\BRperiod}%
\def\br@Exodus{Exo\BRperiod}%
\def\br@Leviticus{Lev\BRperiod}%
\def\br@Numbers{Num\BRperiod}%
\def\br@Deuteronomy{Deu\BRperiod}%
\def\br@Joshua{Jos\BRperiod}%
\def\br@Judges{Jdg\BRperiod}%
\def\br@Ruth{Rut}%
\def\br@ISamuel{\BRbooknumberstyle{1}Sa\BRperiod}%
\def\br@IISamuel{\BRbooknumberstyle{2}Sa\BRperiod}%
\def\br@IKings{\BRbooknumberstyle{1}Ki\BRperiod}%
\def\br@IIKings{\BRbooknumberstyle{2}Ki\BRperiod}%
\def\br@IChronicles{\BRbooknumberstyle{1}Ch\BRperiod}%
\def\br@IIChronicles{\BRbooknumberstyle{2}Ch\BRperiod}%
\def\br@Ezra{Ezr}%
\def\br@Nehemiah{Neh\BRperiod}%
\def\br@Tobit{Tob}%
\def\br@Judith{Jdi}%
\def\br@Esther{Est}%
\def\br@IMaccabees{\BRbooknumberstyle{1}Ma\BRperiod}%
\def\br@IIMaccabees{\BRbooknumberstyle{2}Ma\BRperiod}%
\def\br@Job{Job}%
\def\br@Psalms{Psa\BRperiod}%
\def\br@Proverbs{Pro\BRperiod}%
\def\br@Ecclesiastes{Ecc\BRperiod}%
\def\br@SongofSongs{Son\BRperiod}%
\def\br@Wisdom{Wis\BRperiod}%
\def\br@Ecclesiasticus{Ecu\BRperiod}%
\def\br@Isaiah{Isa\BRperiod}%
\def\br@Jeremiah{Jer\BRperiod}%
\def\br@Lamentations{Lam\BRperiod}%
\def\br@Baruch{Bar}%
\def\br@Ezekiel{Eze\BRperiod}%
\def\br@Daniel{Dan\BRperiod}%
\def\br@Hosea{Hos\BRperiod}%
\def\br@Joel{Joe}%
\def\br@Amos{Amo}%
\def\br@Obadiah{Oba}%
\def\br@Jonah{Jon}%
\def\br@Micah{Mic\BRperiod}%
\def\br@Nahum{Nah\BRperiod}%
\def\br@Habakkuk{Hab\BRperiod}%
\def\br@Zephaniah{Zep\BRperiod}%
\def\br@Haggai{Hag\BRperiod}%
\def\br@Zechariah{Zec\BRperiod}%
\def\br@Malachi{Mal\BRperiod}%
\def\br@Matthew{Mat\BRperiod}%
\def\br@Mark{Mar}%
\def\br@Luke{Luk}%
\def\br@John{Joh}%
\def\br@Acts{Act}%
\def\br@Romans{Rom\BRperiod}%
\def\br@ICorinthians{\BRepistlenumberstyle{1}Co\BRperiod}%
\def\br@IICorinthians{\BRepistlenumberstyle{2}Co\BRperiod}%
\def\br@Galatians{Gal\BRperiod}%
\def\br@Ephesians{Eph\BRperiod}%
\def\br@Philippians{Php\BRperiod}%
\def\br@Colossians{Col\BRperiod}%
\def\br@IThessalonians{\BRepistlenumberstyle{1}Th\BRperiod}%
\def\br@IIThessalonians{\BRepistlenumberstyle{2}Th\BRperiod}%
\def\br@ITimothy{\BRepistlenumberstyle{1}Ti\BRperiod}%
\def\br@IITimothy{\BRepistlenumberstyle{2}Ti\BRperiod}%
\def\br@Titus{Tit\BRperiod}%
\def\br@Philemon{Phm\BRperiod}%
\def\br@Hebrews{Heb\BRperiod}%
\def\br@James{Jam\BRperiod}%
\def\br@IPeter{\BRepistlenumberstyle{1}Pe\BRperiod}%
\def\br@IIPeter{\BRepistlenumberstyle{2}Pe\BRperiod}%
\def\br@IJohn{\BRepistlenumberstyle{1}Jo}%
\def\br@IIJohn{\BRepistlenumberstyle{2}Jo}%
\def\br@IIIJohn{\BRepistlenumberstyle{3}Jo}%
\def\br@Jude{Jde}%
\def\br@Revelation{Rev\BRperiod}}
Edited by Logan West