If you know the password for the account, can you not add it to TB as a separate account?

That's the point really. Gmail delegation exists so that others can manage an account without having to know the account password, and without full power over the account. My web searches suggest that it is possible to configure Thunderbird to access a delegated gmail account as a delegate, using the credentials of the delegee rather than those of the account owner. I just haven't yet found a recipe that works.

I'm using Thunderbird 78.6.0 on a fully up to date Windows 10, in case that is relevant.

I now see the difference; this is the first I've heard of delegated accounts. Since you have already set up the delegation and it works in gmail, where does the process fail if you follow these instructions? You would of course enter imap.gmail.com on 993, SSL/TLS, OAuth2 authentication, and smtp.gmail.com on 587, STARTTLS (or 465, SSL/TLS), OAuth2 authentication. The User Name is you@gmail.com\localcomm@gmail.com.

There is a bug where OAuth2 sometimes isn't shown in the authentication drop-down, so you have to click Advanced config. in the manual config. window and set it in Server Settings and Outgoing Server (SMTP).

Thanks for the further info.

When I try to complete the setup by selecting Done I am first prompted for my password, which is fine, and then I am prompted for the password for the account being delegated. Now I know the password, but I will be setting up someone else with delegated access and I don't want them to have to know the password.

The only variation from your post is that when I select SSL/TLS as the SMTP SSL protocol it changes the port to 465.

You definitely shouldn't be asked for the delegated account's password, so I wonder if TB's OAuth routine is not capable of handling delegated accounts. The instructions on various sites may have worked for 'normal password' authentication, but not the newer OAuth2. Perhaps it will work in TB if you try to access the delegated gmail account from a non-gmail, normal password account. If none of that works, you could add the delagatee's address as a forwarded address in gmail settings.

I corrected my last reply to STARTTLS security on port 587, although SSL/TLS on 465 also works for gmail.

Thanks for the suggestion to use a non-gmail account.

Unfortunately when I try granting access to such an account I get the message "Error: Google account address you have specified is invalid.". The form in which you specify the e-mail address does say "Specify a Google Account holder to access your account.".

I may be trying too hard. It is perfectly possible to manage e-mail as a delegate via a web browser. I'm only anticipating that some users (like me) prefer Thunderbird or other e-mail clients and trying to cater for them.

That said, if someone does come up with a recipe to get Thunderbird to work for a delegated gmail account please do share.