When we investigated the issue, it turned out that the field was disabled in those companies which had a Payables Transaction Approval workflow active; I did additional testing and the Payables Batch Approval workflow does not have the same effect.
It turns out this is a known bug with Microsoft (quality issue 91267) which only occurs when Mutlicurrency Management is not registered:
Once Multicurrency Management is registered in the Registration window (), the field on the Payables Batch Entry window will be enabled:
It is now recommended that Microsoft Dynamics GP be implemented with Multicurrency Management registered, but this is a long time user of Dynamics GP who has never had it registered. Just after I wrote this post, a second client encountered the same issue; again a long time user of Dynamics GP.