Back to the original question of the thread, the only way I know of to get practice reps on C13 greens is to play unlimited tourneys, either CC or the standard WGT weekly / monthly unlimiteds.
What I prefer to do is play practice rounds on T12 greens, which give you all the same features as the practice greens (putt preview, mulligans) but you can pick the greens you want to work on. Then when I play on C13s I make small adjustments to the power (a bit less) and break (a bit more).
I'm sure there are other ways to improve putting but that has worked well for me.