I have used internal routing on a steel mudguard, it worked.
Only problem is that you can't exactly removed the mudguard.
No worries on removing the mudguard...what did you use to hold the cable on the inside of the guard?
I just shove it inside the channel of the steel mudguard, and a bit of superglue.
@Mjosh started
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I have used internal routing on a steel mudguard, it worked.
Only problem is that you can't exactly removed the mudguard.