Created attachment 33240 [details]
Example of formula with labeled arrow

Created attachment 33241 [details]
Desired labeled arrows

according to http://qa.openoffice.org/issue_handling/workflowcharts/
RFE_issues_by_QA.sxd I reassign this issue to requirements and set the status 
to New

Labeled arrows are indeed extensively used in chemistry.

I will add here some signs often used in *chemistry*:

 1. Equilibrium:
    (aka double half arrows for chemical equilibrium)

    simple: dual arrowheads: A <----> B

    complete:
                k1
          _____________\
    Start -------------  Product
          \     k2

    where k1, k2 are the equilibrium constants (are omitted sometimes)

 2. Reaction progression:
    one of the reaction less likely / unlikely

                k1
          _____________\
    Start      --------  Product
               \   k2

    especially k2 is omitted

 See e.g. on wikipedia: http://en.wikipedia.org/wiki/Chemical_equilibrium
 (although the notations there are very basic, yet more than OOo can do)

 For another image with equilibrium constants, see:
 http://goldbook.iupac.org/SVG/C01480-1.png and
 http://goldbook.iupac.org/SVG/F02463.png

 This one uses --> arrows, but half arrows (---`) are also common
 and sometimes preferred.

 3. Broken arrows:
    (I did NOT find them)

    ---X--->, ---//--->, ---/---> and the double arrow versions


Well, TEX has a wide range of useful additions that are currently lacking in OOo.

Just to extend the arrows-issue in chemistry, I append some TEX-elements
   Table 132: chemarrow Extensible Accents
      \autoleftarrow{abc}{def}
      \autorightarrow{abc}{def}
      \autoleftrightharpoons{abc}{def}
      \autorightleftharpoons{abc}{def}
   These symbols are all “reverse accents†in that the two arguments serve,
   respectively, as a superscript and a subscript to the arrows.
   In addition to the symbols shown above, chemarrow also provides
      \larrowfill,
      \rarrowfill,
      \leftrightharpoonsfill, and
      \rightleftharpoonsfill macros.

For an extended list of TEX-symbols, see
http://www.ctan.org/tex-archive/info/symbols/comprehensive/symbols-a4.pdf.

I already added these comments to issue 80616, although that issue should
probably handle different things.

To annotate arrows, you could use csup and csub.  At least, in OOO 3.3 Math, this is possible.

Example: rightarrow csup{abc}

However, the current problem is that the arrow is not elongated to adjust itself to the width of center superscript or subscript contents.

There are two (ugly) workarounds:
1. Construct your own (broken) arrow from Unicode characters:
   ― U+2015 for horizontal line
   → U+2192 for right arrow
   Example:
    X "―→" csup{abc} csub{ijk} Y
   use as many horizontal line chars as necessary!

2. Go to Wikipedia's sandbox (http://en.wikipedia.org/wiki/Wikipedia:Sandbox), edit the page, type in
<math> \xrightarrow{abc} </math>
and show the preview. Wikipedia will then generate a temporary png image for the math formula that you could save and use in the document :-/
But you'd better save the Wiki math expression somewhere if you want to "modify" it later and you don't have to type everywhere from zero.

Off-topic:
For other possibilities in Wikipedia, the URL is
http://en.wikipedia.org/wiki/Help:Displaying_a_formula