As far as I can tell (without my hardware right now, but looking at runtime code), I see no reason why Serial.print(x,BYTE) should limit you to 7 bits... 8 bits should work just fine.
I tried to use Serial.print(x, BYTE) with x>127 and I got 63 in the PC port (?) :'(
By code, it's possible to convert a 8bit value in two 7bit values but It would easier send a 8bit value.
byte x=150, x0, x1;
x0 = x & 127;
x1 = (x >>7) & 1;
I would like to send a byte without making this conversion.