In that case
the old method should still work. You can set
wallet.relayfee = (lambda: 0)
in the Qt console, and then the GUI lets you do whatever you want. Create even zero-fee txs.
But towards the end of that GitHub thread they say that this method doesn't work anymore. TheButterZone suggested a specific order in which you create the transaction to be able to sign it in
this comment. That apparently doesn't work either.
I tried the latest master branch containing that PR but it can't seem to sign the lowest possible 0.1sat/vB transaction at first.
But after manually adding 1sat to the absolute fee, it let me sign the transaction.
Seem to be a minor issue with fee rate calculation since it's too small.
On a side note, since it's merged, we can expect this in the next release.
How exactly did you get the server to accept and sign your transaction?
Too bad (a good thing actually now that there'll be 2), electrum.labrie.ca beat you to it.
I will connect to this server later today and see if it accepts sub 1 sat/vByte transactions. Thanks for the tip.