As of 2015, lucky loser in the WTA tournaments is still determined as the highest ranked player. See 2015 WTA Official Rulebook (internet archive):
C. DRAWS / 1. Singles Main Draw / a. Composition
v. Lucky Losers
The criterion for determining Lucky Loser status is determined
first by the highest ranked players (in descending
order) who have lost in the final round of Qualifying. (The
ranking used to determine the Lucky Loser order is the
same ranking used to determine the Qualifying seeding.)*
If more Lucky Losers are required, the same procedure is
followed for players who have lost in the second-to-last
round of Qualifying, in descending rank order.
UPDATE
The rules changed in 2016. Here is an excerpt from 2016 WTA Official Rulebook (internet archive):
C. DRAWS / 1. Singles Main Draw / a. Composition
v. Lucky Losers
... If there is one vacancy in the
Main Draw before Qualifying is completed, then the order of
the two (2) highest ranked Lucky Losers shall be randomly
drawn, and thereafter the order shall follow the Lucky Losers’
rankings, unless there are two (2) or more withdrawals at the
time Qualifying is completed in which case the size of the random
draw will be the number of withdrawals plus one (1)