Counting number of references using environment or special bib-entry field

Question

Suppose, that we write a survey. Somehow we need to count a number of reviewed papers. The simplest way is to create a counter, say, refsRev and add 1 at each \cite.

I created newcommand \rcite, which wrap \cite and adds 1 to refsRevS counter. However, it would be great to have more general decision.

One possible way of problem-solving is to create a special field in bib-entries (a field note is used for another purposes). Then if this field exists for a bib-entry (or we can check a value inside an entry), count the reference, say within citenum.

I've tried to implement another possible way of problem-solving: to create a special environment. However

1. citenum, which works fine with biblatex is broken for biber
2. I do not know how to save a number of current citings (without duplicates) at the beginning of environment and at the end of environment

As a MWE we can use a MWE given here with some modifications. Perhaps, it is possible to use somehow assoccnt package as it is done by egreg for counting chapters here

Here is my MWE:

MyBook.tex:

\documentclass[a5paper,10pt,twoside,openany,article]{memoir}
\usepackage[backend=biber,bibencoding=utf8,sorting=none,]{biblatex}[2014/06/25]

\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
    @misc{A01,
        author = {Author, A.},
        year = {2001},
        title = {Alpha}
    }
    @misc{B02,
        author = {Buthor, B.},
        year = {2002},
        title = {Betta}
    }
    @misc{C03,
        author = {Cuthor, C.},
        year = {2003},
        title = {Charlie}
    }
    @misc{D04,
    author = {Duthor, D.},
    year = {2004},
    title = {Delta}
    }
    @misc{E05,
        author = {Euthor, D.},
        year = {2005},
        title = {Eelta}
    }
\end{filecontents}
\addbibresource{\jobname.bib}
\usepackage{etoolbox}
\usepackage{totcount,assoccnt}

 % http://texblog.org/2012/04/16/counting-the-total-number-of/  
\newtotcounter{citenum} %total counter for references %broken
\def\oldcite{}
\let\oldcite=\bibcite
\def\bibcite{\stepcounter{citenum}\oldcite}

% attempt to create an environment
\newcounter{refsRev} %counter for total citings in environment countRefs
\newcounter{atBeginOfCountRefs} % save a current number of citings at the begining of env
\newcounter{atEndOfCountRefs} % save a current number of citings at the end of env
\newenvironment{countRefs}{
    % 1) save number of references at the begining and at the end of environment
    % 2) calculate the differense between them
    % 3) add to refsRev the current values of counters 
    %\addtocounter{refsRev}{\numexpr\value{atBeginOfCountRefs}-\value{atEndOfCountRefs}\relax}
}


% new cite-like command, seems to be safe, i.e. with the same properities as \cite[]{} 
\newcounter{refsRevS}
\newcommand{\rcite}[2][]{\cite[#1]{#2}\addtocounter{refsRevS}{1}}

\begin{document}
Here we need to mention that there are \therefsRev $\:$  references in some parts of text, which are wrapped by our environment countRefs.

This document contains \total{citenum} references (possibly with multiple citations). %broken
\section{First}
Some text \autocite{A01}.
  \begin{countRefs}
Here we begin review papers.
Some text \autocite{B02} = \rcite{B02}
Some text \autocite{B02}. % this value should be deleted
\section{Second}
Some text \autocite{C03}.
Some text \autocite{D04}.
Here we temporary stop review papers.
  \end{countRefs}
Some text \autocite{A01}.
The current number of refsRev papers is ... % 3 papers
\section{Third}
\begin{countRefs}
Continuation of papers review.  
Some text \cite[p.2~]{E05} = \rcite[p.2~]{E05}
Some text \autocite{D04}.
End of papers review.   
\end{countRefs}
The current number of refsRev papers is ... % 4 papers
The current number of refsRevS papers is \therefsRevS.
\end{document}

The question is how to get one of two mentioned decisions to count survey papers?

UPT: It would be great to have an opportunity to add by \rcite a citing into a counter, which is used for the environment, i.e. to mix these two approaches.