If you held a fixed extra weight each time, it would disguise your real weight from other eyes. Downside is that it would alter the BMI calculation and make your weight seem higher, but you could just let others know that it is not your REAL weight. Using 7 or 12 (or even just 1) kg would make the increase less easy for others to puzzle out. Or go for the double bluff and say that you have when you haven´t to preserve an accurate BMI. After all what you weigh is your secret to tell, or not, as you wish.
On Wii Fit Plus you have the option to set a password to use your mii (therefore being able to hide your weight from others). I have had mine set from the beginning so anyone who wants to access my mii would have to know my 4 pin password!