Hi, its me again. 
According to the BBB SRM and schematics the VDD_3V3B is powered from a buck converter (TL5209) which is sourced by the SYS_5V rail. The TL5209 can power up to 500mA, but there are some more peripherials powered over this rail (UART0, JTAG, MMC0, ethernet).
So there is only 250mA provided at the headers (P9_03 and P9_04).
I want to make a custom cape with an LCD display which needs like 400mA max at full brightness and power it over these 3.3V pins. Is it safe to do this when I just use the UART0 and don´t use the JTAG, MMC0 and ethernet port on my BBB? This would simplify my layout for the cape very much.
Cheers