Helmat has sent in a patch for the doas utility which allows doas to launch interactive logins as another user, similar to the way "sudo -i" or "su -l" work. This makes it a little easier/faster to set up an interactive shell.
The interactive login can be launched by running "doas -S" or "doas -S -u user", and is basically a shorthand for "doas -u user -- su -l".
The new -S flag will be available in the next release of doas, which I hope to publish in the next month. Thank you to Helmat for the clean patch and updated manual page.