unlang to customize error message

[Stefano Zanmarchi]

According to MS-CHAP-V2 standard in case of authentication failure the
Failure Packet should contain the following text in the Message
field: "E=691 R=... ..." .
Freeradius does send "E=691" if send_error is set to yes
in mods-enabled/eap, but this may cause (as stated in the comments)  some
clients not to work.
This is sadly the case with iOS 9.3.1 supplicants.
Luckily we have a table relating MACs and OSs of the devices of our users.
Is it possible via unlang to have Freeradius conditionally send "E=691" (in
case authentication failure) based on a sql query?
I believe this is quite tricky, but it'd really help us a lot.
Thank you,
Stefano