no fault found.
I traced the issue to a few lines of dial code further down in the _custom file.
exten => _X.,1,NoOp(Do Not Disturb: ${SIPPEER(${CHANNEL(peername)},donotdisturb)})
exten => _X.,1,Set(SIPPEER(${CHANNEL(peername)},donotdisturb)=yes);
exten => _X.,1,Set(SIPPEER(${CHANNEL(peername)},donotdisturb)=no);
These little buggers were picked up from http://docs.acsdata.co.nz/asterisk-cisco/sippeer-options.shtml and were not supposed to be in the file beyond experimenting. As those codes match any dialled number, the correct dialplan flow was not being followed.
With them removed, the outbound route is inaccessible when required.