Is it possible to add entries to the bibliography based on keyword using Biblatex/Biber and within the document code?

Update: This is now possible with biber 2.14/biblatex 3.14, both in their respective development folders on github [1][2]:

\documentclass{article}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@book{new-stuff,
    author          =   {Watt, Brian},
    title           =   {Happy Times with Penguins},
    publisher       =   {Harvard University Press},
    address         =   {Cambridge, MA},
    year            =   1995,
    pagination      =   {section},
    keywords        =   {happy}}
@book{den-coll,
  author                =   {Till, Jr., Dennis E.},
    booktitle           =   {Penguin Land and Further North: Human Influence},
    title               =   {Penguin Land and Further North: Human Influence},
    publisher           =   {Oxford University Press},
    year                =   2008,
    address             =   {Oxford and New York},
    keywords            =   {penguin, human}}
@book{old-stuff,
  author                =   {Harvey, Jr., Dennis E.},
    booktitle           =   {Penguin Land and Further North: Human Influence},
    title               =   {Penguin Land and Further North: Human Influence},
    publisher           =   {Someone \& Daughters},
    year                =   1567,
    address             =   {Oxford},
    bookpagination      =   {paragraph},
    keywords            =   {penguin}}
\end{filecontents*}

\usepackage[defernumbers=true]{biblatex}
\addbibresource{\jobname.bib}
\DeclareSourcemap{
  \maps[datatype=bibtex]{
    \map{
      \step[nocited, final]
      \step[fieldsource=keywords, notmatch=happy, final]
      \step[entrynull]
    }
  }
}

\begin{document}

\cite{old-stuff}

\nocite{*}
\printbibliography

\end{document}

This removes entries during parsing the data sources which are \nocited or which don't have the keyword.

This is an old question, and it doesn't look like there is a really satisfactory answer. But here's a solution that involves a perl script just for fun.

Remember to run it with pdflatex --enable-write18.

\documentclass{article}
\usepackage{filecontents}
\begin{filecontents*}{allentries.bib}
@book{new-stuff,
    author          =   {Watt, Brian},
    title           =   {Happy Times with Penguins},
    publisher       =   {Harvard University Press},
    address         =   {Cambridge, MA},
    year            =   1995,
    pagination      =   {section},
    keywords        =   {happy}}
@book{den-coll,
  author                =   {Till, Jr., Dennis E.},
    booktitle           =   {Penguin Land and Further North: Human Influence},
    title               =   {Penguin Land and Further North: Human Influence},
    publisher           =   {Oxford University Press},
    year                =   2008,
    address             =   {Oxford and New York},
    keywords            =   {penguin, human}}
@book{old-stuff,
  author                =   {Harvey, Jr., Dennis E.},
    booktitle           =   {Penguin Land and Further North: Human Influence},
    title               =   {Penguin Land and Further North: Human Influence},
    publisher           =   {Someone \& Daughters},
    year                =   1567,
    address             =   {Oxford},
    bookpagination      =   {paragraph},
    keywords            =   {penguin}}
\end{filecontents*}
\begin{filecontents*}{citekeyword.pl}
use strict;
use warnings;
use Text::BibTeX;
my $keyword = "happy"; # set keyword here
my $bibfile = Text::BibTeX::File->new("allentries.bib");
my $keywordcitefile = "keywordcites.tex";
open(my $fh, '>', $keywordcitefile) or
    die "Could not open file '$keywordcitefile' $!";
my $keywords;
my $entry;
while ($entry = Text::BibTeX::Entry->new($bibfile)) {
    next unless $entry->parse_ok;
    if ($entry->get("keywords")) {
        $keywords = "," . $entry->get("keywords") . ",";
        if ($keywords =~ /,$keyword,/) {
            print $fh "\\nocite{" . $entry->key . "}\n";
        }
    }
}
close $fh;
\end{filecontents*}
\immediate\write18{/usr/bin/perl citekeyword.pl}
\usepackage[defernumbers=true]{biblatex}
\addbibresource{allentries.bib}
\pagestyle{empty}
\begin{document}
\input{keywordcites.tex}
\cite{old-stuff}
\printbibliography
\end{document}