The only relevant MIBs would be
CISCO-ISDN-MIB.my, CISCO-ISDN-MIB-V1SMI.my,
ftp://ftp.cisco.com/pub/mibs/supportlists/as5300/as5300-supportlist.html
These do not have NFAS specific info,
but they should still provide the D channel
information.
Looks like there is a bug to improve
some of the NFAS data retrieval; CSCds10171.