The problem here is that you can't use titlept as a custom field name. You have to use the fields that are defined by biblatex. You might use usera for example. Otherwise biblatex is not able to recognize your input fields.
Example:
\documentclass{article}
\usepackage[
backend=biber,
]{biblatex}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@book {foo,
author = {Foo, Francis},
usera = {My custom title},
title = {All about Foo},
year = {2011},
location = {Footown},
}
@book {bar,
author = {Bar, Bernie},
title = {Barstory},
year = {2000},
location = {Barcity},
}
\end{filecontents}
\addbibresource{\jobname.bib}
\renewbibmacro{maintitle+title}{%
\iffieldundef{usera}{ % if usera is undefined
\iffieldsequal{maintitle}{title}
{\clearfield{maintitle}%
\clearfield{mainsubtitle}%
\clearfield{maintitleaddon}}
{\iffieldundef{maintitle}
{}
{\usebibmacro{maintitle}%
\newunit\newblock
\iffieldundef{volume}
{}
{\printfield{volume}%
\printfield{part}%
\setunit{\addcolon\space}}}}}
{\printfield{usera}\newunit\newblock}%
\usebibmacro{title}%
\newunit}
\begin{document}
\nocite{*}
\printbibliography
\end{document}
For more information about custom fields in biblatex you should read section 2.2.4 of the manual.
Currently these custom fields are supported:
name[a–c] list (name)
Custom lists for special bibliography styles. Not used by the standard bibliography
styles.
name[a–c]type field (key)
Similar to authortype and editortype but referring to the fields name[a-c]. Not used by the standard bibliography styles.
list[a–f] list (literal)
Custom lists for special bibliography styles. Not used by the standard bibliography styles.
user[a–f] field (literal)
Custom lists for special bibliography styles. Not used by the standard bibliography styles.
verb[a–c] field (literal)
Similar to the custom fields above except that these are verbatim fields. Not used by the standard bibliography styles.
titleptas a name of a field. You have to use the fields that are defined bybiblatex. You might useuserafor example. Otherwisebiblatexis not able to recognize your input fields. Other possibilities can be found in section 2.2.4 of the manual. – Thorsten Jan 10 '12 at 13:17