The mishna[3] states that a kohen with blemishes on his hands should not perform Birkat Kohanim because this will cause people to look at him. Rashi[4] comments that the Divine Presence rests upon the kohen’s hands during Birkat Kohanim, and seeing them could therefore be dangerous. Tosafot[5] argues convincingly that the Divine Presence accompanied the kohanim only in the Beit HaMikdash. Rather, he argues, the point is that when people stare at a peculiarity of a kohen, it hinders their concentration. Either way, the blemish does not disqualify the kohen per se, but creates a problem that the Rabbis decided must be avoided.
It is questionable whether a cast meets the requirements of a “wonder”[6] that distracts people. On the other hand, it might be somewhat of a curiosity to see Birkat Kohanim done with a cast, and perhaps it would draw improper attention. Realize that even one whose hands are colored (e.g., by a dye) does not do Birkat Kohanim.[7]
Let us thus examine other elements of the issue.
The gemara[8] states that even one whose blemish would ordinarily preclude him from doing Birkat Kohanim may do so if the people of his town are familiar with his condition. The Shulchan Aruch[9] rules that this applies to one who has been in the city for thirty days. After people have become accustomed to the condition, they will not be tempted to look at him during Birkat Kohanim.[10] Thus, after the cast has been on for thirty days (assuming you are in a place where you are a “regular”), there is no issue.
There is another major factor that may allow you to do Birkat Kohanim. The Shulchan Aruch[11] rules that if the local custom is for the kohanim to drape their tallitot over their heads, then even one with blemishes on his face or hands may do Birkat Kohanim, provided that the blemish is thereby covered.[12] (It does not suffice that the members of the congregations usually avoid looking, because people are more likely to be drawn to look when there is a blemish to see.[13]) Thus, if in following your local minhag your tallit will cover enough of your arm so that the cast will not be visible (which is usually the case), you may continue doing Birkat Kohanim.
What if the local minhag is not to cover that much, but you want to do so? If your sleeve covers the entire cast (even if you usually do not wear long sleeves), there should be no problem, as how you wear your clothing is a matter of personal preference; it is not related to minhagim that affect halacha. A question does arise, however, if you cover the cast with a tallit, contrary to the minhag. The implication of the Shulchan Aruch is that this would not enable you to do Birkat Kohanim, but it is unclear why not. The Igrot Moshe[14] explains that it is not because of a prohibition to change the minhag. Rather, differing from the practice because of a blemish likely draws attention to it. It stands to reason, then, that if you are in a community where some kohanim cover more and some cover less, you could cover more now to keep the cast out of view.
In summary, you should evaluate your situation based on the above guidelines. If you come to the conclusion that you should not do Birkat Kohanim, be aware that according to some authorities, you should walk out before Birkat Kohanim so that people will not suspect that something is wrong with your kohen status.[15] Some say that this is not necessary because your visible blemish “informs” others why you are not doing Birkat Kohanim. There is no conclusive ruling on the matter.[16]