"For ARRL Award and Contest purposes, all transmitters and receivers comprising a station used for a specific contact must be located within a 500-meter diameter circle."

The current rules: http://www.arrl.org/dxcc-rules
(Basis rules 9.b)
So according this rule it is not allowed to use the WebSDR for the DXCC award.

This rule makes no sense.
1) A two way QSO depends on BOTH receive and transmit capabilities. Improving only one does not help making more DX QSO's.
Suppose you use a low dipole for RX and TX. Using a large beam for receive only will not improve your transmitted signal.
The low dipole then still limits your achievable two way QSO's.
That is fully in line with what we experience using the WebSDR in Maasbree.
2) strangely it is allowed to have both receiver and transmitter on a remote location like Maasbree.
But that improves BOTH receive and transmit capabilities. And that will give you an advantage over other hams.

The only reason why a remote receiver makes a difference is local man-made noise and QRM.
A remote receiver doesn't give you any advantage over other hams, but is solves a disadvantage!