I think one possible disadvantage of the lighting poles mentioned are that they are plain conical tapers, going from the preferred base diameter to 4.5" at the top. However, many softwood trees I've looked grow that way, so it's probably not a huge concern. Lightmart have specification sheets with engineering data published by each item, but they are in a format for highway engineers, not naval architects. Data is data, though, and I'm sure the specifications can be deciphered. Also of interest is that some of their offerings come with a base fixture that looks as if it would take very minor work to convert to a mast step.

They also offer steel and fiberglass poles. From my observations, the price of steel is not markedly better than aluminum. Fiberglass is not likely to be desirable, but offers some interesting possibilities as mandrels for custom-built carbon spars.

I don't know if these have been mentioned before, but here are a few sources for round/tapered masts in the US. They do not show up in the information section, so I thought I'd toss them in. Feel free to delete this if it is redundant information.

Rig-Rite offers masts from 4" diameter to 8" OD in 6063 T6 alloy. The largest length is 33'6" (sorry for my non-metricness), and has a .220 wall thickness. Rig-Rite does not offer much pricing information, and I haven't found them particularly good at answering emails or being forthcoming on the phone, however. Their web site is archaic and not particularly easy to navigate, but pertinent information on round masts can be found here:

http://www.rigrite.com/Spars/Rig-Rite_Spars/RigRite_Round_Masts.html

Lightmart is a commercial lighting pole manufacturer, and offers some intriguing options, including free shipping, choice of a range of powder-coated colors at no additional charge, and a clear and concise pricing structure. They offer tapered poles in lengths to 40', with diameters to 10" and wall thicknesses to .312" in their "hurricane rated" series. They use 6063 T6 as well.

Their range can be found here: http://www.lightmart.com/Aluminum-Light-Poles.aspx

They also feature an "economy range" that appears as if it would be perfectly adequate if one wished to experiment with smaller boats.