I'm a little concerned that the thinwall aluminium tube might not be strong enough for the job once it is extended like that. It might be a good idea to reinforce the tube section that is close to the powerhead - that is the one that will experience the greater bending moment.
My old (1980s) Echo has a steel tube outer, rather than aluminium tube. If you could replace the first aluminium tube (the one at the powerhead end) with a steel one of the same dimensions, it would be twice as strong.
When the pole saw is running it is a dangerous object, and to me the idea of risking its support breaking under load, is quite unattractive.
