I'm going to start by disclaiming that my z50 knowledge is very limited, but working with Toshiba in general is not.
As you've already googled, I'm going to guess you've done many of the below things, but here is an ordered list of the usual "at home" options, before you end up having to ask Toshiba how specifically to do it for a z50.
1) Pull the cmos battery. From memory however, Toshiba has decided to be wonderful and make sure cmos doesn't lose its passwords during a battery pull. You can try the "pull the battery for an hour" method though, just in case i'm wrong there.
2) Short the bios reset pins. Thats generally what a professional shop will do. I tried to find a picture of where they are on a z50, but I can't find anything. Unfortunately Toshiba is not consistent in the slightest in the location of these pins. They do, however, on some models print "reset" in tiny tiny letters on the board. It's entirely possible you'll be lucky and reviewing your spare MB can find where it is.
2.1) On this note, its not always labeled (which makes this while likely the same solution Toshiba would use, extremely difficult to guess on at home), and sometimes requires the computer to be started. Even within the Tecra line of laptops there are a bunch of different possible scenarios of which pins you need to short to reset. Some are near the mini-wifi adapter, some are near the memory. It's gonna depend on the laptop.
3) You can try some of the backdoor passwords for BIOS systems. I am including the link to that, I'm not sure how much this is still practiced by manufacturers, but maybe we'll be lucky http://searchenterprisedesktop.techtarget.com/tip/How-to-Bypass-BIOS-Passwords
4) Beyond that, the answer is the Toshiba Repair Centers. While not an ideal answer I'm sure which is why you came here instead, they at least have the expert specific for that model. I unfortunately, don't have a manual for the z50.