I've just committed the changes to the repo. The modified package loading hook doesn't seem to cause issues in my tests.

Savetrees package redefines lot of internal LaTeX commands, so it is no surprise that it crashes tex4ht. As the changes it does aren't useful in conversion to HTML, an easy solution is to use conditional inclusion.

Another possibility I've just found is to block the loading of the package from tex4ht. It seems that it is possible to call an action at \usepackage call, tex4ht has list of packages which request it. It is possible to add a package to this list in tex4ht-4ht.tex:

\<use package\><<<
savetrees,>>>

\<add to usepackage\><<<
\def\:temp{savetrees}\ifx \@currname\:temp
% code to be executed
\fi

>>>

these configurations are added to usepackage.4ht, which is included every time \usepackage command is used. Now we need to block package loading.

I've found nice macro:

% http://tex.stackexchange.com/a/39418/2891
\newcommand{\dontusepackage}[2][]{%
\@namedef{ver@#2.sty}{9999/12/31}%
\@namedef{opt@#2.sty}{#1}}

the problem is, it needs to be called with the same package options as \usepackage in the document, otherwise we would get option clash error message. I've took a look at the LaTeX and tex4ht.sty sources and it seems that the options aren't saved in any macro. In fact, I've found following definition in tex4ht.sty:

\let\:onefilewithoptions\@onefilewithoptions
\def\@onefilewithoptions#1[{%
\let\:temp\@reset@ptions
\def\@reset@ptions{%
\let\@reset@ptions\:temp
\edef\TivhTcats{%
\catcode`:=\the\catcode`:%
\catcode`@=\the\catcode`@}%
\catcode`\:=11 \catcode`\@=11
\a:usepackage
\TivhTcats
}%
\:onefilewithoptions#1[}

The original \@onefilewithoptions has following parameters

\def\@onefilewithoptions#1[#2][#3]#4{%

where the second argument are package options. We can change the tex4ht redefinition slightly:

\let\:onefilewithoptions\@onefilewithoptions
\def\@onefilewithoptions#1[#2]{%
\let\:temp\@reset@ptions
\def\@reset@ptions{%
\let\@reset@ptions\:temp
\edef\TivhTcats{%
\catcode`:=\the\catcode`:%
\catcode`@=\the\catcode`@}%
\catcode`\:=11 \catcode`\@=11
\def\:currentoptions{#2}
\a:usepackage
\TivhTcats
}%
\:onefilewithoptions#1[#2]}

as you can see, new macro is defined, holding package options. We can then define macro for blocking package loading like:

\newcommand{\:dontusepackage}[2][\:currentoptions]{%
\@namedef{ver@#2.sty}{9999/12/31}%
\@namedef{opt@#2.sty}{#1}
}

What do you think? Is that change safe?