define_webjump can of course provide a completer, but the full range of sophistication of the completions
system is not available to webjumps because define_webjump does not provide access to related mechanisms
like $match_required.  $match_required must be used if one wants to use the completer keyword $get_value
to transform the result item.  The current best workaround is probably to provide a handler function for
the webjump which performs the transformation.

issue231 may possibly be a specific instance of this problem.