Sadly the manuals we have are not all that good on systems, but interestingly enough that is the quote from our limitations section:
QFE Operation
QFE operation is prohibited on the main aircraft altitude system due to the fact that EGPWS data are based on QNH.
Only thing about barometric inputs in the technical documentation is this single sentence about the look ahead terrain feature:
Look-ahead terrain alerts are based on the airplane’s position, barometric altitude, vertical flight path, and ground speed.
All in all it seems barometric inputs are used directly by parts of the EGPWS we have installed, other installations might differ of course. On our old 733s which did not have GPS input to the FMC the EGPWS had their own internal GPS and were indeed independent of barometric inputs.