How to bake a veiny cake?
I wouldn't bake them in, for a start, as I wouldn't be confident of what the bootlaces would do in the oven. Thinking about it, a possible way of going about it is to 'thread' the bootlaces through the cake before you ice it. 

Cut the cake in half (vertically), get a big needle like a darning needle or something similar, and (carefully) thread a few laces through, cutting them off flush with the surface, then stick the cake back together with icing or jam and pipe on the outer icing. The tricky thing will be getting them to follow curvy paths.

The other thing you could do is use said needle (or perhaps a chopstick) to poke little tunnels through the cake, then pipe red jam into them. This would perhaps give a nicer result - it might be a bit odd to be eating a slice of cake and then to suddenly be chewing a rubbery bootlace.