To answer my on question:
Under Ubuntu 16.04 and 18.04, with the BOINC manager installed from the repository, the manager runs as a daemon, and it has it's own environment, so the SWAN_SYNC=1 setting should placed in it's own configuration file at

/lib/systemd/system/boinc-client.service

This file should be edited as 'root'.
In the [service] section of that file there should be a line containing:

Environment="SWAN_SYNC=1"

It should be exactly like the above (the SWAN_SYNC should be capitalised, and the quotation marks should be there.)
The host should be rebooted to make this change take effect.

Thanks to Rod4x4 for this solution!

Hey Zoltan, what do you mean by edited as root? Does this mean you change User=boinc to User=root?

Hey Zoltan, what do you mean by edited as root? Does this mean you change User=boinc to User=root?

Try running the text editor as root; for example:
sudo gedit /lib/systemd/system/boinc-client.service

Good to hear that Zoltan and Rod4X4 worked it out how to get SWAN_SYNC=1 working on repository versions of BOINC. My posts in the Linux thread seems to have provided the necessary dialog and testing for the solution to be exposed.

So now both the Seti Tbar and repository versions of BOINC can benefit from the improvement in using SWAN_SYNC. Project benefits from increased throughput.

Hey Zoltan, what do you mean by edited as root? Does this mean you change User=boinc to User=root?

Try running the text editor as root; for example:
sudo gedit /lib/systemd/system/boinc-client.service

That's what I meant.

/lib/systemd/system/boinc-client.service
This file should be edited as 'root'.
In the [service] section of that file there should be a line containing: Environment="SWAN_SYNC=1"

NOTE:
1. Ensure the Environment= line is before the ExecStart= line.
The Environment= line is best placed as the first line after the [Service] heading.
2. Performing an update on the boinc-client package may need this line re-added after the update. (have not tested this)

First of all, thank you very much to all kind contributors to bringing this to work, in this and other threads.

Now, a step-by-step practical way to apply this in a few minutes (for Linux rookies like me and above)
I've just applied successfully this method in three different computers with already running GPU tasks without the need for them to end, and all tasks have restarted normally after applying the new configuration.
All three computers are running under Linux 16.04.5 LTS and repository BOINC versions (7.6.31 x64).

-1) If there are tasks running: Enter BOINC Manager - Activity - Suspend. Exit Boinc Manager. This will checkpoint and stop all running tasks
-2) Open a Terminal window, and execute the following commands:
-3) sudo killall boinc
-4) sudo gedit /lib/systemd/system/boinc-client.service
-5) Add the following line immediately below [Service] tag: Environment="SWAN_SYNC=1"
-6) Save the change and close gedit instance. Return to Terminal window
-7) sudo systemctl daemon-reload
-8) sudo /etc/init.d/boinc-client restart
-9) Close Terminal window
-10) Enter BOINC Manager - Activity - Run

That's all. Previous running tasks will restart from its stored checkpoint, but a full CPU thread will be assigned to every GPU.

One last advise:
Please, be caraful. This configuration will bring the whole system to its maximum performance.
More power = more heat. It Might be a good idea to check system fans and heatsinks to be free of dust...
And I use Psensor program from Linux repository. I recommend it to check temperatures, fan levels, and several other interesting indicatives.

Thank you all!

my gtx 1080 GPU is now on 99% usage with linux!

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Cruncher/Learner in progress.