Hey Blue,
Found this at line6 website: [Invalid or Expired Link Removed]
The short of it is the Relay G30 uses their "DC-1G" power adapter, which is 9V DC, 500mA, Center Pin: (-) (2.1mm). In other words basically the same as a Boss PSA.
Line 6 recommends the Voodoo Labs pedal power, other supplies will work but 'are not supported'.
So the one spot should be fine. It's total is a max 1700mA for everything connected to it. I doubt the receiver actually uses 500mA. You could find out exactly how much by putting a multimeter on it, or contact Line 6 via their site, it seems helpful.