yeah, i plan to use the origional steel headtube, as i didnt fancy the strength of bamboo there, or fitting a headset. i might try and "wrap" the headtube in bamboo to make it look like its made from it.
It will be disproportionately large compared to the rest of the frame.
Bamboo has thick walls. To sheath a headtube you will need a piece of bamboo that is twice or better as large as the rest of the bamboo in your frame.
With mine I build up the headtube to the size of the rest of the tubes with the hemp that I used to lash the frame together.