Ok that's my try:
\documentclass{beamer}
\usepackage{tikz, etoolbox}
\usepackage{etoolbox}
\newcount\mycount
\def\mytempcoord{}
\def\mytempopa{}
%
\newcommand\doatpos[3]{%
\bgroup
\renewcommand*{\do}[1]{\ifnum\mycount<2
\edef\mytempcoord{\mytempcoord ##1%
\ifnum\mycount<1
,
\fi
}
\else
\edef\mytempopa{##1}
\fi
\advance\mycount by 1
}
\docsvlist{#2}\global\advance\mycount by -3
\begin{tikzpicture}[remember picture, overlay]
\node[anchor=#1, opacity=\mytempopa] at (\mytempcoord) {#3};
\end{tikzpicture}%
%
%
\egroup
%
}
\begin{document}
\frame{
\doatpos{west}{0cm, 3cm,.5}{This is text \rule{1cm}{1cm}}
\doatpos{west}{1cm, -3 cm,.7}{This is text a\rule{1cm}{1cm}}
}
\end{document}
I basically wrote a parser which divides this three item comma list in such a way that it can be used by an ordinary sequence like my TikZ overlay thing here.
You might find that your third argument, the pseudo z-variable, needs no dimension. Well yes. Involving a certain key handler here could be useful.
However, my opinion is different. I rather would introduce a new argument (in braces). That would make things much easier. (My very personal view is that mimicking the behavior of completely different systems is rather pointless.)

Same output, but no etoolbox:
\documentclass{beamer}
\usepackage{tikz}
\def\mytemparray{}
%
\newcommand\doatpos[3]{%
\bgroup
\def\mytemparray{{ #2 }}
\pgfmathparse{\mytemparray[0]} \edef\mya{\pgfmathresult}
\pgfmathparse{\mytemparray[1]} \edef\myb{\pgfmathresult}
\pgfmathparse{\mytemparray[2]} \edef\myc{\pgfmathresult} %also possible \pgfmathsetmacro
\begin{tikzpicture}[remember picture, overlay]
\node[anchor=#1, opacity=\myc] at ( \mya pt, \myb pt) {#3};
\end{tikzpicture}%
%
%
\egroup
%
}
\begin{document}
\frame{
\doatpos{west}{0, 3 cm, .5}{This is text \rule{1cm}{1cm}}
\doatpos{west}{1cm, -3 cm,.7}{This is text a\rule{1cm}{1cm}}
}
\end{document}
This alternative uses pgfmath for list-processing. Since pgfmathparse is very expansion unfriendly i decided to hard-code and put those three elements in distinguished macros. This solution is again very specific and inflexible, but i think it's more obvious how to process more items in the list.
z? Are you able to project 3-dimensional PDFs? – Qrrbrbirlbel Dec 03 '12 at 02:22zcoordinate should define order of visibility of objects. That's why there's no dimension in my MWE (minimal wanted example) – Crowley Dec 03 '12 at 09:32pst-absposortextposand/or TikZ using the specialcurrent pagenode. – Qrrbrbirlbel Dec 03 '12 at 15:39\tikz[remember picture,overlay]\node[anchor=center] at (current page.center) {\includegraphics{foo}};would do. Thezpart can be achieved by defining appropiate layers. – JLDiaz Dec 14 '12 at 18:50