Back in april 2012 Robert Blackstone asked some questions about indexing
http://www.mail-archive.com/ntg-context@ntg.nl/msg63134.html
i.a.:

For a book in the area of musicology I would like to make one register with 
entries of different kinds, distinguishable by their appearance in the  
register (or index or whatever).
The index entries will mostly be authors, including composers, musical terms 
and musical compositions. The titles of the compositions and the foreign words 
are set in italic characters throughout in the text of the book.
What I would like is to be able to distinguish all entries according to whether 
they are treated in the text, are shown in a musical example, or are mentioned 
only in a foot- or endnote.

In many books page numbers indicating illustrations are set in italics. 
Pagenumbers indicating foot- or endnotes often have a suffix "n".

His other question were resolved, but this one remained unanswered, apart from that two other users also wanted to do exactly the same thing.

I would like to do something very similar and was wondering: Has this been addressed/solved since then?

\defineprocessor [backpart] [style=slanted,right=n]
\defineprocessor [Figure] [style=\tf,color=red,left=f]
\defineconversionset [backpart:pagenumber][][backpart->o]
\setupuserpagenumber[way=byblock]
\setuppagenumbering [location=none]
\setupheadertexts[{\convertedcounter[userpage][numberconversionset=number]}]
\setupnote[footnote][location=none]
\useMPlibrary[dum]
\starttext
\startbodymatter
\startchapter[title=A chapter]
  \index{aaa}This is in the main body.\footnote{\index{bbb}This is a footnote.}
\stopchapter
\startchapter[title={Another chapter}]
  \index{aaa}This is in the main body.\footnote{\index{bbb}This is a footnote.}
  \startplacefigure[location=default,
      title={Body illustration\index[Figure->]{ccc}}]
    \externalfigure[dummy][]
  \stopplacefigure
\stopchapter
\stopbodymatter
\page
\startbackmatter
\startchapter[title=An appendix]
  This is in the backmatter.\index{aaa}\footnote{\index{bbb}This is a footnote.}
  \startplacefigure[location=default,
      title={Appendix illustration\index[Figure->]{ddd}}]
    \externalfigure[dummy][]
  \stopplacefigure
\stopchapter
\startchapter[title=Notes]
  \placenotes[footnote]
\stopchapter
\startchapter[title=Index]
  \placeregister[index]
\stopchapter
\stopbackmatter
\stoptext