Well, this one was recently asked by another brewer. I would question whether the yeast cake is the best route for 2 reasons:
1) Wyeast says it is not a good idea, because the percentages of the yeasts and bugs changes from the original pitch.
2) The age of the slurry is probably old enough to be suspect (either autolysis or Brett eating the sach and others present, just because it can).
That said, what the heck, if you think you have a "Mother Funk" working and want to send it on to a new generation, go ahead and send it. I have an over 5 year old Solera in a 5 gallon bourbon barrel that has had probably 5 or more iterations of Flanders Red blended within it. I use it to blend with a "new" Flanders (5 gallon batch) and an intermediate Flanders (varying amount available and held in glass), each time using proportions of each that result in a taste that I prefer, scaled up to become a 5 gallon serving batch. The new Flanders is typically the bulk of the blend and each time I start the new Flanders with a new pitch (just 1 smack pack). So give it a try if you want, it likely will be good beer regardless, but you may want to help out that sach yeast a bit by pitching some new Wyeast Roeselare.