As the truss of the fsck command indicated that the fsck was stuck in a loop:

kpread(3, 0x000000011012B090, 1024, 0x0000000000F7B800, 0x0000000000003DEE) = 1024
kpread(3, 0x000000011016B3D0, 1024, 0x0000000000F7AC00, 0x0000000000003DEB) = 1024
kpread(3, 0x000000011016B7F0, 1024, 0x0000000000F7B400, 0x0000000000003DED) = 1024
kpread(3, 0x000000011016B3D0, 32768, 0x0000000000F7A000, 0x0000000000003DE8) = 32768
kpread(3, 0x000000011012B090, 1024, 0x0000000000F7B800, 0x0000000000003DEE) = 1024
kpread(3, 0x000000011016B3D0, 1024, 0x0000000000F7AC00, 0x0000000000003DEB) = 1024
kpread(3, 0x000000011016B7F0, 1024, 0x0000000000F7B400, 0x0000000000003DED) = 1024
kpread(3, 0x000000011016B3D0, 32768, 0x0000000000F7A000, 0x0000000000003DE8) = 32768
kpread(3, 0x000000011012B090, 1024, 0x0000000000F7B800, 0x0000000000003DEE) = 1024
kpread(3, 0x000000011016B3D0, 1024, 0x0000000000F7AC00, 0x0000000000003DEB) = 1024
kpread(3, 0x000000011016B7F0, 1024, 0x0000000000F7B400, 0x0000000000003DED) = 1024
kpread(3, 0x000000011016B3D0, 32768, 0x0000000000F7A000, 0x0000000000003DE8) = 32768

the fsck was stopped (using CTRL-C) and the metasave captured and replayed in-house. Whilst checking it in-house it was found that by running a full fsck with the 'nolog' option followed by a full fsck, the fsck would complete and the filesystem mounted. By checking the metasave in-house first it could be determined what the fsck command would fix and if any files would be moved to lost+found, so that this could be taken into consideration prior to doing these steps on the production server.