Could a string of large offshore wind towers support a Hyperloop tube?
It's a well defined [tm] engineering problem well within the physical capabilities of available materials and methods, so, yes, they could.
However, the practicality of integrating the two very different systems is vanishingly small.
Offshore wind turbine towers are spaced at very large distances apart compared to the structures that would be necessary to support a practical hyperloop track. Adjusting WT tower placement so their foundations and part of their structures could be jointly utilised is conceptually feasible, but the percentage of hyperloop supports suitable for joint use would be a small percentage of the total, and the added constraints of routing and location imposed on both systems would be likely to be significant.
A hyperloop track could be used to provide support for a large number of smaller wind turbines along the while route, but the economics of wind turbines greatly favour the largest and tallest market proven* systems available.
(*There are always larger and taller systems seeking to prove themselves).
It seems likely that a "hyperloop" could be economically be built in a land based tunnel than on an above surface marine platform. Security, and mechanical stability are both significant requirements. While subterranean tunnel systems face challenges from earthquakes, this is a well known and understood (if not well predictable) phenomenon and paid response sensor systems are liable to render a tunnel system no more dangerous than a marine above surface platform.
From 'memory' - figures could be researched and added if of interest:
China operates a large network of high speed rail - mostly above ground, but still subject to derailment effects from earthquakes and other land movement. Speeds are around (from memory) 150+ kph for the Z trains and 200-250 kph for the D trains, with faster speeds in some sections. While well below hyperloop speeds the challenges are similar. I have heard of the very occasional report of Chinese rail incidents but they seem to be as rare as one would hope.
Over middle and longer distances a D train achieves trip times about half that of a car over the same general route. (I've traveled in D trains a few times).