To follow up on myself, the rights issues have been fixed.
The key step was making the amanda user member of the backup group.
Notice: it is necessary to logout an existing amanda session when
rebuilding, as otherwise the original group membership will
persist (and hence the original error message, see below bottom).

Below procedure will make you happy on SPARC Solaris 7
(appropriately GNUified, of course):

bash-2.00$ /usr/local/sbin/amcheck daily
Amanda Tape Server Host Check
-----------------------------
Holding disk /Disk2/AMANDA: 2667734 KB disk space available, that's plenty
NOTE: skipping tape-writable test
Tape DailySet11 label ok
NOTE: info dir /usr/adm/amanda/DailySet1/curinfo: does not exist
NOTE: it will be created on the next run
NOTE: index dir /usr/adm/amanda/DailySet1/index/192.168.0.73: does not exist
Server check took 0.061 seconds

Amanda Backup Client Hosts Check
--------------------------------
ERROR: 192.168.0.73: [can not read/write /usr/local/var/amanda/gnutar-lists/.: 
No such file or directory]
Client check: 1 host checked in 1.104 seconds, 1 problem found

(brought to you by Amanda 2.4.4p1)

There are only minor config issues left to be resolved.

Thanks, everybody!

Eugen Leitl wrote:

Andrea Dell'Amico wrote:

Don't do that, neither. Assign the group "backup" to the amanda user,
chown root /usr/local/sbin/amcheck, and then rerun it as amanda.

I've done what Gene Heskett said:

# cat /etc/group
...
backup::303:amanda
...
# cat /etc/passwd
...
amanda:x:400:303:Amanda Operator:/Disk2/amanda:/bin/bash
...

As user amanda:
bash-2.00$ tar zxvf amanda-2.4.4p1.tar.gz
...
bash-2.00$ export LDFLAGS="-L/usr/local/lib -R/usr/local/lib"
bash-2.00$ cd amanda-2.4.4p1
bash-2.00$ ./configure --with-user=amanda --with-group=backup
...
bash-2.00$ make
...
bash-2.00$ su
# make install
...
bash-2.00$ id amanda
uid=400(amanda) gid=303(backup)

bash-2.00$ amcheck daily
bash: /usr/local/sbin/amcheck: Permission denied

I've tried diverse suggestions from other posters (thanks!),
but so far can't get anything other than a verboten, or the usual
'port not secure' shinola.

Thanks, I'll keep trying.