I think the point here in using HDPE is the survival of the whole tower in an impact that would cause either the tower to bend or the navigation equipment to break.
Prototyping aside (HDPE as earlier said is easy to work with) one can make the aluminium tower in such a way that it would break in certain, replacable joints in a heavy impact. But you have to ask yourself if you want to go through the designing process of such a thing. If aluminium bends, for the professional rider, it may or may not cause a problem as in the next bivouac he will have the chance to choose between a googlion of nav towers.
Safetywise, I would prefer to have a design that is good. Material is irrelevant in this case, as HDPE if the design is wrong will damage you in an accident. So from this point of view as an engineer I would not go to HDPE for that and only for that reason. There are many other reasons described above which make it an attractive material and if it is used in conjuction with other materials then may be the user can get the best of both worlds.
As far as the UV problem, I have installed d1000 HDPE pipes and they are still there, biogas pipes are mainly HDPE, water pipes are mainly HDPE etc. I would worry about other things first than UV protection. You can go and buy black HDPE which contains carbon black and you will be ok for the rest of the bike's life.
Mechanically speaking, nobody said that HDPE has higher Young's Modulus than Aluminium, load to weight ratio etc... But you have you always have to ask yourself what you need in any given design. The sentence ''nobody has done it before'' or ''they are not using this material for this type of design'' proves nothing to me and I am an Engineer... Let alone if you are not one and you are just a member who reads these two sentences
. Check the plastic bash plates. They are there for a reason. Check the CF bash plates. They exist too . Aluminium tanks vs Plastic tanks etc ... ... Some you win some you loose
To get back in two the Nav Tower subject, I think a combination in the right places of HDPE, Aluminium and may be CF would be the best. Aluminium could be used where the assembly needs to be stiff (the head part for example), HDPE in the joints or other stressed members in a crash in order for them to give a bit of room in the unlikely event
. Carefully designing such a thing would be a great success in my opinion. Especially if the ''break away'' parts can be replaced very fast as nobody wants to sit there forever after a crash replacing parts in ther navigation tower.
I think the problem, if you are not the one with the right equipment in your hands, are the suppliers. You have to find somebody to do the HDPE, the Aluminium and the CF. Even Aluminium may need two or three suppliers to get it right (at least here in Greece
) and that, may prevent some people from using more than one material. Dont forget that TheAntiPaul had already a good design and just changed the material. And this specific design happens to be flat plates. That is a very good example of a simple and good design as you can just buy yourself the HDPE sheet and cut it, drill it and off you go... Complex designs are more difficult to perfect and if looks is what you want then I dont think you can make HDPE welds and bends that look half as good as an anodised aluminium part or a CF part...
But then again if you want good looks you go to a hair salon