I did the same thing last year. I can look up the exact amounts tomorrow.
As far as process goes, I split the rhubarb in two parts, adding one part for the last 5 minutes of the boil, while freezing the other half and adding the thawed fruit after primary fermentation was complete. The reason for this was simply my lack of an idea how to best express the rhubarb's character in the finished product. Needless to say, this process did little in the way of expanding my knowledge on the matter.
Also, I think it is a misconception to assume that freezing sterilises stuff. I guess that it should reduce the number of critters, perhaps to a negligible order of magnitude, but there seems to be very little reliable and broadly applicable info on this.
However, people put fruit - frozen, heat-pasturized, or not treated at all - into beers all the time. I think that beer (that is, fully fermented wort, preferably hopped) is quite resilient and, given that some microorganisms such as brewer's yeast are still around, an environment that makes it difficult for intruding microbes to take hold.
EDIT: The day before brewday, I sliced 1.5kg of rhubarb, of which 1kg went into the freezer (and then into the fermentor during the peak of fermentation) and 500g were mixed with some sugar (which should help break up the fruit and release the juiced) and left for 24 hours. After those 24 hours, the rhubarb was pressed and the juice collected (it wasn't terribly much, as I recall) and added into the boil for the last 5 minutes. (All amounts for a 5gal batch)
I remember the rhubarb character as quite pleasant, a bit acidic (as expected), but perhaps a bit too indistinct. Overall, I did not like the beer too much, which I mostly blame on yeast choice (Mangrove Jack's "French Saison", probably the same as Lallemand's "Belle Saison") and the buckwheat I included in the grist, which had a rather "dirty" taste. However, I still think that rhubarb does well in a saison.